if the Sony feeds the empeg Aux In, then when the empeg is out of the car, you have no music

Not sure I'd agree with you there; with a bit of circuitry, you can sort this out. Domenic recommended something like this a LOOOOOOONNNNNGGGG time ago somewhere on this board.

This is exactly my situation in the Laguna we own; a Philips head unit with no Line In, but a stereo Line Out. My original (mistaken) intent was to feed the empeg through the Philips, so we could retain CD capability and get away with not having to buy an external amp. No way, Jose - instead, I needed to feed the empeg with the Philips. I did this by the use of a 4 pole, two-way relay with gold contacts (and BIG springs!), so that when the empeg is inserted, the audio source becomes the empeg by energising the relay with the remote antenna output. Empeg out, the amp is fed with line-level transformers from the normal high-level outputs of the Philips. Takes a bit of fiddling to set it up initially, but it works OK.

As regards noise... what noise?

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