JimmyK

Update: Got past the relay by just using the lines downwind of the IPDM (from the connectors on the IPDM). I confirmed that this works by purposely shorting that same solenoid to car ground (not the battery) and heard the beeps loud and clear.

So far I have tested two of the four lines, but of course the tests are not comprehensive because it's really really hard to get to all the harnessing to rattle it. Still no luck unfortunately.

As ridiculous as it sounds, I can rent a trailer and tow my car around while listening for the shorts with a few DMMs so I can at least narrow things down.