My two cents saycheck the grounds. I see a lot of electrical problems inaircraft due to corrosion where components ground to the airframes. I don't know how your car is grounded but it may be something to look at.

Hows the battery and alternator when the lights and air conditioner are on, basically when they are under load.

 

If you take a reading on the stud where the battery cable attaches that will be giving the Alt output.