Here come the pictures:
First, the final product, The Monsoon Box:
This is the "output" side. The little DC jack takes 12V power (actually, it powers a 6V regulator, so probably anything above 8V would work), these RCA jacks have about +6V referenced to the ground supplied by the 12V plug. The other (input) side has just two RCA jacks, they don't see the +6V.
Now, the schematic:
Here's the basic idea:
Take a 5V or 6V voltage regulator, attach three diodes to the output. In the second picture I included the diode package so people can see the part number. Notice the polarity indicator on the diode. This way the 5V flows to the RCA jacks, but the signals on those jacks don't come back thru the diodes so they can't cross to the other signal lines.
Take a ground loop isolator and splice the outputs from the diodes into one side of the RCA outputs from the GLI. One diode goes to L+, one goes to R+, and one goes to both the L and R -. If I was doing it again, I would wire up 4 diodes to the voltage regulator output rather than joining the L- and R-. It doesn't actually matter since other parts combine the - lines into a common one.
No, I didn't just tape everything together. Those are but connectors that I covered with electrical tape.
So, does it work? I don't know. A test version of this circuit worked great. This box produces ~5.8V on the output side and nothing on the input side. Before I installed in the car, I was adding a relay to control the 12V input to this box. While testing the wiring for the relay, I smoked my m1-atx. So, I'm ****ed now and I have to deal with that.