[DanEC]

Well, the headlights probably pull up to 20 amps on high beam, less on low beam. That combined with the 23 amp fan and tail lights, parking lights, dash lights, etc would probably be marginal with a stock alternator at idle. It could also be that your alternator is a little on the weak side. It might be a good idea to pull the alternator and take it down to an auto parts store that can test it and see if it's putting out like it should. I would also check all your grounds to make sure they are good. And, I would check the temperature of your wiring to the fan when it's running. If it warms up considerably, possibly it's too small of gage for that fan load and pulling an excessive load on the alternator.