Been a while since I looked around carefully in the back hatch, but the only ground wires that come to mind are the short ground wires that attach the two sections of the hinge to each other. If those are broken, everything will still work, but it is never a good plan to conduct electricity through moving parts (hinges and latches) because if they are well greased, they don't conduct very well, and if they aren't, they conduct ok but the electrical flow makes them corrode.

I would not expect your locks or windows to rely on the ground in the rear hatch (though I've seen stranger things). All I can tell you on those other locks and switches is that you need to take a voltmeter or 12V test light and start at the hot side of the fuse and go all the way to the motor until you find the problem (making sure that the motor or lock actuator are grounded).

If you are unclear on how relays work and how to check them, we can run through that for you. Come to think of it, I wrote a long discussion on that a couple of months ago, either here or on the S-70 board.