To answer two questions in one, Sony make an RCA switching unit (XA-39 IIRC, but don't quote me on that) that allows you to use two RCA level outputs into one amplification unit. It's not cheap though, at around 80ukp last time I looked. A better idea might be a DIY unit as I've suggested in the past for utilising existing RCA CD-changer inputs on head units. Something like this. For 'head-unit rca inputs' read 'amp inputs', and for 'CD rca outputs' read 'existing radio outputs'. I've not actually tested this, but can only assume it'll work. The resistor values will probably need to be dropped a bit, and might need a bit of fine tuning to get a decent balance between the volume levels for each unit. If you're running a 4 channel set-up, utilising both RCAs on the Empeg, then you'll obviously need to duplicate this circuit for each RCA pair, tying the signal grounds to each other across each circuit, and splitting the orange Empeg to each relay.

If it's a 4-channel setup, then diode D1 *definitely* needs to be in place on each circuit to kill any back EMF floating around. You might still get switching noise - if anyone does actually try this and that is the case, give me a shout and I'll suggest a few add-ons that should get rid of it.

That's assuming the existing head unit has 4 RCA outs. If not, then I'd definitely recommend a Kenwood KVD-840 speaker to phono adaptor. They're a little bit more expensive than most other spkr>phono adaptors, but work an infinite mount better, and give you 4 RCA outs, where most other units just handle 2.

Dominic

Cambridge Car Audio
http://www.cambscaraudio.co.uk