Second the engine bay request. I can post pics of my '85 engine bay for comparison on vacuum routing (system is 100%). Did he pull vacuum lines out of the body, too, or just leave those unplugged?
The rear window sounds like a flaky motor, the brushes inside are probably worn. Or, a broken wire inside the hinge area. I think you can rule out the switch at least. I say that, because it acts that way on both switches so it's not just a flaky front switch, which can happen too.
To confirm this you can test the wiring at the motor by pulling the back off the rear switch to reveal the wires. Use your tester to find which one gets 12v when you toggle the front switch. If it is getting 12v, but the window motor is not moving, it pretty much confirms the motor. The motor can be replaced separately from the regulator. Good used ones are on eBay or at your local scrapyard. You can see this link to see how the wiring looks behind the switch:
http://www.ifixit.com/Guide/Mercedes+W123+Power+Window+Regulator%2C+Rear+Replacement/20428#s56347