Body ground point.
Usually found on the firewall, between the battery and the heater valve, but really close to the battery. Should have a few wires attached to the firewall with a phillips-head screw. Probably rusted up.
Your battery negative cable should have two wires. The usual heavy wire (usuallly attached to the engine via one of the starter bolts) and a lighter gauge wire which connects to the body ground point.
As Wild said - you NEED it.
Given all your electrical gremlins, if you can get hold of an infrared thermometer, check your engine temperature with that. Unless you absolutely know the engine was overheating (running badly, spewing coolant, all the usual signs)
Cheers
Mike