Defi Oil Pressure Gauge - Need Advice

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Finished installing a Defi oil pressure gauge (52 mm metric Racer version) yesterday. Connected the gauge harness per the instructions provided by Defi ... except had no success with the "illumination" wire. (Evidently connecting this wire will result in the gauge dimming at night when the headlights are turned on). After consulting the FSM, tried tapping into the white "illumination (-)" wire that runs from the M81 connector (main harness) that plugs into the Bose radio. The radio display dims when the headlights are turned on ... and I confirmed that the white wire controls the dimming (i.e., the dimming feature on the radio display did not work after cutting the wire). After connecting the gauge wire and the white wire (using a butt-end connector with a solder insert), the dimming feature on the radio was restored, but the gauge did not dim when the headlights were turned on. In other words, a complete waste of time that involved removing all of the trim pieces and the audio unit in order to get to the M81 connector. So I'm now left with a gauge that works (... actually looks and works great), but without the dimming feature. Has anyone who has installed a Defi gauge managed to identify what wire to tap into in order for the dimming feature to work? I know that one member tapped into a wire marked "illumination" that ran off an aftermarket HU with success, but that doesn't apply in my case. Any help would be much appreciated. Incidentally, I'm in a 2013 Touring Sport with the Bose audio system, but without the nav unit. Also, I need to do a reset on the passenger-side airbag after disconnecting it to remove the audio unit ... but I think that someone has posted instructions on how to do the reset.