impala 2002 windows

We have a 2002 chevy impala. all the windows stopped working at once.
we checked the fuses, replaced the master switch on drivers door. Changed the master relay for windows. Still no windows work. You hear a slight buzzing from the switch when you push down hard.

We took the doors all apart but could not find the ground wire. Someone told us to check that . Do you know how to find the ground wire, or what else we should check to fix this window problem?