I can't ever recall having seen a wireless FM modulator, but personally, I'd tend to steer clear of them either way.

Some head units we see over here have either a CD changer input on phonos, that you could switch using a relay as I've mentioned before now (Sony also make a ready built unit designed for this sort of thing), those that don't, you can usually buy auxilliary input adaptors for. Offhand, I know Alpine, Sony (XA-107) and Pioneer (CD-RB20) do them, so I'd imagine the same applies in the US and most other parts of the world.

In most cases though, the Empeg's probably going to end up being used as the primary source - that being the case, probably the best bet would be to buy a small, decent quality amp, run the system off this, using the Empeg's pre-outs, then feed an RCA (or speaker level input, through a speaker to phono adaptor) from the original head unit in the Empeg's auxilliary input.

Having said all of that, if anyone is determined to go down the wired FM modulator route and has any trouble tracking one down, then we can always supply them :-)

Dominic

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