Another detail I thought of is this:
If you have the pin-out, you notice there should be one wire going to the diagnostic link. If you wire that up to the appropriate port in the diagnostic link, a tech could enter into the MCS with the SDS computer, but I don't think there would be much he could do, but it's worth a shot.

We're talking about 2 different things in regards to that key, I'm just curious if it works or not, it makes the radio display brighter, not dimmer, and isn't adjustable.

In addition to the display not getting dimmer when the lights are switched on, I assume if you lower the brightness of the instrument cluster with the adjusting knob nothing happens to the radio brightness?
I believe the code for the instrument panel light circuit is 58, have you checked that one and seen if it varies with the instrument panel adjustment knob being turned?

Gilly

PS Just realized you say the "ADJ"/"LGHT" option isn't present with the headlights on. This feature I believe may only be present with the headlights OFF, as it's intended for use during the day if sunlight is hitting the MCS faceplate, as it tends to get "washed out" in direct sunlight. Try it with the headlights off once.