power window imtermittent problems

I have a 68' vette with power windows that are having problems. The window works when it wants, and I am not sure why. Does anyone know where the ground for the window motor is, as I think that may be part of the problem? Anyone that has anyother ideas on what could be the problem feel free to let me know.

check the connections at the switch first. you can buy some electrical contact cleaner spray and spray the connectors and the switch itself. over the years they build up with dust and dirt and the contacts fail. i believe a place like radio shack carries the stuff. spray it on and it dries in seconds, but i cleans anything electrical.