Anyway, I was thinking why not use ordinary CAT5 or similar for the mic
cable and/or the separation cable? Has anyone done this, and if so a
wiring diagram?


I have done this to my IC2800 with no problems whatsoever. the radio is with
the spare wheel and the mic plugs into a socket mounted in the dash with the
cable (CAT5) being routed around the car.

Also I have extended the display lead in a similar way (but due to not
having a 6 way plug/socket for the dispaly out came the 'iron and heatshrink
and more CAT5) again no problems and due to the fact that I couldn't be
bothered to measure, the cables are long enough to mount the radio in the
back of a huge camper and still use it from the front seat!