Have a 92 mercury with a 1.6L. Roll the windows up and down the idle drops. Let go it evens out. Same with fan motor. Turn it on. Drop then back up when off. Same with headlights. Good battery and alternator. What could it be scotty?

IF battery and alternator are loaded tested by a pro and are both good, then wiring, either power or ground, has minor shorts dragging stuff down