In article ,
spouted forth into uk.rec.audio.car...
The thought had crossed my mind, and it's "plan B" at the moment - but my
preference would be a solution that relied on copper, and not air, to get
the audio into the head unit. ;-)

An FM modulator does use copper.

It commonly uses the fm frquency usually reserved for Radio 2 and
changes your audio signal from whatever device you are connecting, into
an fm frequency radio signal (it wires inlines and is not wireless in
any way).

Sound quality varies from average to poor, and at best is going to very
good FM radio quality. I've used one, and it was average, would try to
avoid if you are going to be trying to play very high bitrate MP3's or
navigation voice data through it.
--
Carl Robson
(The poster formerly known as Skodapilot)
http://www.bouncing-czechs.com