Hi all, just bought a gen4. Was horrified to realise it didn't have an AUX in (mine has the non-mcintosh stereo). I did some googling and found nobody really had a solution, so i thought maybe my solution might be worth sharing.
I found the circuit diagram online last night, and wanted to hijack the AM or FM inputs to become AUX, however they were both mono as best i can tell from the circuits. So i cut the ribbon cable between the CD and the rest of the unit, and soldered a cable with a 3.5mm plug on the end onto it. Anyway, works great - just leave any old CD in the player, and connect the aux in to your phone, and voila. Here's a photo just before i soldered it up, attached.
Thought it might be helpful to someone, way cheaper than one of those custom fascia replacements that allow you to fit a DIN unit.
Having said all that, i'm now considering removing the storage fascia above the standard unit (where a satnav option can go) and putting in a head unit there, is that a common thing to do?