inside air, outside air, its your choice and with your situation either would probably be just fine, you could take it from the garage, but if it were me, i would not, and not because of any really good reason other than if i were going to bother running pipe, then i would run it all the way.

What kind of heat do you have now, ductless? Jackfre makes a good point about having the new ducts tested. Does anyone know what the industry standard is for new duct leakage? In addition to that, keep in mind that what does not leak now, will in the future. Ducts outside of the living space are a bad compromise. When you get a leak in them, you add a power vent to your house. If in a supply duct you blow air out is one spot and suck it in in unpredictable places. If in a return, you suck air in from around that and then blow it out from leaks in the house envelope elsewhere. If you don't have returns in every room or other means of air return, you get pressure differentials between rooms that do the same thing. The trouble with those whole-house fans from the 50s is that they typically let a lot of air leak by when not in operation. The shutters don't seal well. You really need something like a Tamarack. I question the utility of a whole-house fan in NJ. I won't dismiss them out of hand because they can be useful in the right climate. A good source of information about what to do for you house might be your local land grant university. They typically have good, climate-specific information for homeowners. You could also try the adjacent states, Penn State, Delaware, NY (Cornell). Building Science Corporation (www.buildingscience.com) tends to be more technical, but I think it will be useful as well. Since it is meant to be nationwide, you have to zero in on your climate and weed out all the stuff for other areas so it is more time-consuming. Power vents suck. They can be a good band-aid for a bad attic, but often not even that.

The condensate line (drain line) from the evaporator above the furnace is probably clogged somewhere. Not knowing what type fittings you have, I couldn't advise on exactly what to do. I would call for service for the drain and get the system checked out as well if it hasn't been for for the 5 years. The system could be icing up as well, but it wouldn't be cooling the home well.

First a question: is this a high efficiency furnace using 'inside' air? If so, why not just run an air supply? O.K., if you're looking for outside combustion air, look at the following link. One item works on negative pressure; the other is a passive device (no fans, etc., but relies on the weight of cooler air to shut off the supply (only vents when furnace/water heater are on). V http://www.skuttle.com/makeupcontrol.html