Ok. The windshield wiper motor, windows and gauges all get their power from a fuse in the the fuse block in the cabin and their ground via the instrument panel ground down by the driver side courtesy light bulb. I'm thinking it and the other parts attached to the rear half of the chassis work because they are getting grounded through the shifter cable back to the battery. All of the grounds for the rear half of the car ground to the chassis sheet metal. The shifter cable is metal and connects to the sheet metal via the shifter and connects to the transmission on the other end which is connected to the engine which is connected to the starter which has the negative battery cable hooked to it. If you turn on a few things for a while and run your power windows up and down a few times then feel your shifter cable, it's probably getting warm, maybe even hot. If you do test this, feel the cable before running anything so you'll have a reference point as far as how warm it is or gets. If your running the engine, it could get warm just from the exhaust pipes.
This is why the braided ground cables, from the frame to the trans or bell housing and from the engine block/head to the chassis(firewall) are so important.