'91 4Runner 4x4 auto with swapped '98 3.4.

At idle if I put it into reverse the battery voltage drops, lights dim. Fair enough. However, if in reverse and if the power windows are up and I press the switch to roll them up, the battery voltage increases and lights get brighter.
Head lights, turn signals even the heated rear window dont have anything like the voltage drop the reversing lights do. And the power windows thing is just weird.

Any ideas?

 

Sounds like you have a gremlin... odd is definitely the right term...

 

I'm only at the novice stages of understanding chassis electrical issues, but maybe this site will help you. Read whatever articles seem relavent:

http://www.autoshop101.com/

 

A very novice guess here.......

The charging system is supposed to generate enough extra voltage to offset voltage load where the battery cannot suffice. Case in point, chassis electricals. Seems to me, if the voltage is spiking like that, maybe the regulator is overcompensating somehow or you're getting voltage bleed when the window switch is activated.

 

I agree hook a voltage reg issue sounds like a possible ans.