Hi, I have just joined and wanted to share this (even though I suspect it has been covered plenty of times in the past). I recently purchased a 20 year old 520i as my daily driver - it was cheap and I expected some "issues" as I have owned an older BMW previously.
I did notice that the "P" light beside the gear shifter was staying illuminated all the time - to be honest I thought perhaps it was supposed to, until my battery went flat. A bit of Googling raised the idea that "sleep mode" wasn't engaging - perhaps due to the HVAC or something.
Anyway, long story short - I didn't want to tear the thing apart but simply turned on the AC to full noise and opened all of the vents - dust and bits of foam (perished linings in the vents?) went everywhere - it clearly hadn't been used in a while. End result, lock car up and return 20 mins later and "P" light has gone out. It's been good for a week now.
