I thought that the P-shift-lock solenoid (or its relay) was triggered by the brake light switch, the logic being that you had to have the brake pedal depressed before you could move the gear lever out of Park. On my S90 wiring diagram (not the same thing I know) there is no relay, and the solenoid receives current directly from the brake light switch when the gear lever is in Park.

For the historically minded, carmakers started putting this in - or gov'ts began requiring it - when Audi started getting bad publicity and a slug of lawsuits when a few of their cars "suddenly accelerated on their own" after being put in D or R. In one case the car went right through the garage back wall and into the owner's swimming pool. All the complainants insisted they were pressing on the brake pedal with all their might when this happened.

Audi conducted demonstrations for the motoring press and gov't officials showing that even with the engine at full throttle and screaming in P or N, the car would only lurch a few inches if the brakes were applied as the trans was dropped into gear. This was rather convincing evidence that the complainants must have been pressing on the wrong pedal.

A high proportion of the complainants were, ahem, "elderly", but Audi went the politically correct route and did not emphasize this when defending themselves, referring only to "pedal misapplication", which was backed up by NHTSA investigations.
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Bob: Son's XC70, daughter's 940, my 81 and 83 240's, 89 745 (V8) and S90. Also '77 MGB and some old motorcycles