One of the door jam switches could be at fault, but just as common, if not more-so occasionally someone turns the instrument panel dimmer control all the way up, and that also has a set of contacts that turn on the interior lamps.

When I teach an introductory electronics class I use that circuit as one of the demonstrations for voltage drop. if you measure the voltage from the ground side of the lamp while it is on, to battery negative terminal, you will get a very low voltage. Its important to note this voltage will NOT be 0volts, but it may be .00XX volts. By going around the car or truck and operating switches that would command the interior lamps to come on, you will see most of them make a difference in the voltage measured to ground. This difference will be as little as .001-002v but they will make a difference if they are working correctly and provide a lower resistance ground circuit. One switch will not make any difference at all. Concentrate on that one first to see if it is holding the light(s) on.

I had the same problem with my truck. On my truck the switches are inside the door as part of the latch. A little WD-40 on the latches and my lights went off. Try that before signing up for John G's electronics course. lol

A lot of these pressure switches jam after a while. If you see someone driving down the road with their brake lights permanently on it is down to the same thing. A switch similar to that on the door lives under the brake pedal and after a while it can become stuck. I had that problem. Trying to lubricate the switch itself to free it up didn't work, I had to buy a new switch from the local main dealer. But it was only a few cent anyway, didn't cost me anything to try and fix the old one first.

Tim, why would you mock the CORRECT answer?

"A little WD-40 on the latches and my lights went off. Try that before signing up for John G's electronics course. lol"

First the poster did NOT specify what kind of truck we are dealing with. If its a Ford, you may have provided an answer for him, IF the switch isnt too far gone. Lubrication does not always restore the operation of one of those switchs, when that happens it still needs to be diagnosed. If "country" has any one of the other brands of trucks out there, your answer does not help him at all.

It gets pretty frustrating at times when learning how to really do the job right, and then sharing that gets disrespected.

Hello again. My truck is a 1999 Ford Ranger. I haven't had time to do any of the things mentioned in here but will give these solutions to my husband this weekend and see if he can fix it. Right now I'm just tired of being in the dark!
Will let you know what finally worked. (if any does!)
Thanks so much for all of you who are trying to figure out my problem for me!

John relax. I didn't say anything was wrong with your solution. I was only saying what worked for me when I had the EXACT SAME PROBLEM. And now seeing that the poster has a Ranger (same as my Mazda B2300), my solution will probably be the easiest and cheapest and least complicated solution. So why not try it first?

Pretty simple reason actually. People waste thousands of dollars on silver bullet parts swaps all of the time, and then when they are unsuccessful they show up at a shop and now they don't have any more money to spend. In fact they often times act as if the effort required to test correctly is some kind of a scam because someone told them something like "all they needed to do was spray a little penetrating oil somewhere". Meanwhile these same people who cut down on our routines have no idea that there is a real way to test and prove what is wrong, and it doesn't even take very long to do it. I don't suppose you have ever been in the position where you spent several hours figuring out a complicated diagnostic situation, only to have the customer then come into the business, after you have proven what the vehicle needs, and be met with an angry and distrusting attitude. I can't tell you how many times someone else has arm chaired a diagnosis, or else under cut the repair pricing once the diagnosis is made when they have not made the investment up front for the education and equipment that was required to identify the root problem. Many times in these cases a customer will treat us like we did something wrong, because they have been misinformed by a third party for whatever reason.

BTW, I am relaxed, and I know you really didn't write that to be confrontational. Just remember this, every time something like that happens in the business, and not on some web forum, it costs a small business real money that can never be replaced. In the end it impacts the employees of the business in both continuing education as well as tooling and equipment that they need in order to stay up to date with the technology that is already on the streets today. Some day, and in many ways that day is already here, cars will routinely be presented at the shops with problems that the shop and its techs are not trained and equipped to solve.

BTW, its the same symptom, not the same problem unless she somehow got your old door ajar switches into her truck. VBG.....

Yeah but a little fluid in a door latch ain't gonna hurt anything.

Many people come on the internet because they can't afford to go to a pro. I myself have been ripped off more times than not and can understand when someone is trying to find a solution before spending outrageous shop rates on a problem they can possibly fix themselves. And yes I know why shop rates are what they are - equipment, training, etc. That doesn't change the fact that sometimes, especially nowadays, people have to make a decision between a tank of gas or spending their last $100 on an expensive repair.

"Yeah but a little fluid in a door latch ain't gonna hurt anything."

Are you sure about that?

Yup.

Hmmmm, and here I was trusting the AMD engineers to know what they were talking about.....

You're about as correct as I am, because neither of us really have a fricken clue.

She/he has the same vehicle and symptom as I have, and I told her what worked for me. It's her/his choice to spend a bunch of money for a pro to look at it or not.

I used about $0.03 worth of WD-40 and it fixed the problem. Yes the problem, not the symptom.

"yawn"....

A number of newer vehicles have added photo optics to the latch assemblies to ensure that the latch is secured in place of contact switches. Lubricants misapplied can coat the sensor and allow dust to accumulate and stick to it causing incorrect circuit outputs issues. So just when we thought it would always be OK to lube the latch assemblies, along comes a car that it isn't.

Your $0.03 worth of lubricant in that case could result in a pretty significant expense to the vehicle owner and you might not even be able to take that car to be repaired to anyone except for the dealer or a shop that chooses to specialize on that make and buy their factory scan tool. The photo optics are part of the vehicle door module and to service it may (probably does) require coding to be input from a scan tool.

If there is one thing this trade teaches us its first not only do we not know everything, the very moment we are sure we can do a routine like lube a latch assembly, along comes a vehicle where we will risk causing a problem if we try to do what has always been considered normal. The bigger problems with that are, nobody ever seems to tell us in advance, we usually find these things out the hard way. Then once we find out about them we get to deal with public attitudes like "You're about as correct as I am, because neither of us really have a fricken clue".

Its a good thing I really do like doing what I do........

Thanks for all the help! I had my husband read all the comments on here and he did the lubricant and so far it has worked! Put the bulb back in and I'm no longer in the dark! I work til 11:00 at night so I really like to have it when I open up the door at night!
I hope it doesn't mess anything up but that's the chance you take. It's an older truck, a '99 and I'm going to get me another one in a year and a half so I'm just trying to make this one do til it's paid for. Times are tough right now and I don't have a lot of money to put out for small details like a light not coming on, I'd just live without it. I love this forum because someone usually has an answer for me. I also use the computer forum when something goes wrong with my computer I don't understand. Both have saved me money and time.
Thanks again guys (gals?) and keep watching this forum and help out when you can. There's a lot of people like me out there who needs your experienced help.

No, thank you.