Headlight switch

I have a 92 f150 with the problematic idle issue. I have fixed consequently every thing possible ie. Map sensor, alternator, battery, fuel filter and fuel pump. I pull out the headlight switch and the engine runs fine but switch to the rear tank and the amp meter in the cab goes up and down and the idle goes to hell. Switch to the front and problem goes away. Does anybody know exactly what all the headlight switch controls and could it possibly be a short going to the rear tank that is drawing power?????? Just looking for any ideas would be great cause like i said I have fixed everything......

looks like your on to somthing