OK, 12V understood. With the ignition in P0 & radio on, the only ECU that should wake up are BECM, and EAS. Filtering the 12V is really only good for Alternator & other engine related noise, which you don't have because engine not running !!
If the Sub feed is wired to an RCA Line connector, then you have potentially un-balanced the feed. I did this with my old Pioneer stereo which had RCA for all 5 outputs. Didn't use speaker outputs !! It worked but there was some low level buzz, albeit not audible with radio turned up. So is the Sub buzzing the same as the doors ?
I still say connect temporary good earths to radio & all amps, bypassing the loom splices (these are hard to locate & the soldering has been know to break down over time). Then report back.
Pete