install AUX input on car audio/navi system
Does it have facility to connect a CD changer? My first thought would be to try patching Aux in through the CD changer connection.

Another option might be to look if you can get your hands on a Saab --> ISO harness adaptor and see if the pin allocation gives you any pointers.

It might just be coincidence, but there's a 20-way connector on the unit, and the ISO C (miscellaneous) connector is also 20-way. (Though I'd not blindly follow the ISO pin numbering if I tried anything with it, but I'd certainly look at it).


  What are (or might be) the connectors ACC, PARK, ILL, ILL-CONT, BU, REMO?


My guesses would be:

ACC - Accessory - +12V swithced by the ignition (stage 1 / 2).
PARK - parking brake (some functions - eg video - might be disabled when the parking brake is on).
ILL - Illumination - lights up when the car's lights are on.
ILL-CONT - (real guess this one...) illumination control (variable brightness if the car dashboard brightness can be varied).
BU - Backup perhaps - constant +12V to keep the memory (eg radio stations) stored when the unit is off.
REMO - (nothing immediately springs to mind on that one).