Washed and waxed the 'wagon, cleaned and fed the leather.
Routed a 4-way adaptor from the lighter socket to under the armrest and ran a cable to power my Sat. Nav. on the right hand side of the instrument pods.
Then it all went wrong.
Bought a Kenwood Bluetooth / iPod head unit, pulled the old head unit and plugged the new one in.
Doesn't work.
It appears that installing an aftermarket head unit in a 156 with Bose isn't quite so simple.
Only one company appear to stock the wiring harness to sort it, and even having looked at the pictures I'm not convinced it'll do the job for me.
(It supposedly takes the line-level output from the head unit to the Bose amp. - but the head unit is only outputting 2 channels, so I'd lose my fader from what I can see)