Electric tailgate window problems

[79Suburban]

I have a 79 Suburban and half the time the rear window on the tailgate won't go down. I put another switch in and it still doesn't work. When the window is up and you push the switch up the volt gauge drops. When you push the switch down the gauge doesn't move and neither does the window. I tried hooking direct power to the down wire and that didn't work either. Then sometimes I'll be driving and push the switch down and the window will go down. Any sugestions? Thanks