I was considering a source selector as an alternative to routing the signals through the EMPEG. I was just going to make one with a DPDT relay controlled by simple diode/transistor logic based on the remote outputs of the OEM CD and EMPEG.

This is what I have done in my install, a large 10A DC DPDT relay with a few diodes and it works perfectly OK, with no noise that I can detect. Cheap, as well.

One of the few remaining Mk1 owners... #00015