brake lights don't work, overhead cab light does. I have replaced bubs/fuse on 2007 Chevrolet Silverado 1500

I don't know where the electrical ground is. what else is there to check?

Check to see if high mounted brake lamp works. That lamp is works directly from stop light switch and does not go through turn signal switch. Not likely a ground problem.
Yes the high mounted brake lamp does work. Do the rear brake lights go thru the turn signal switch?

Yes...but check the bulbs on both sides first..could be blown fillements on 2 bulbs. I would then test at the sockets with a test light while someone steps on brake. Double check fuses. and check to see if a trailer wire harness could have been installed and having corosion issues there.
Thanx for your help I will check both areas