Ok, replaced the cheap-ass speaker-to-line level adapters with a Scosche OEA-4 (4-channel, adjustable). That fixed up the ground loop problems entirely. Before rewiring, I popped open the OEA-4 on a hunch and verified that each of eight lines has a capacitor wired in serially on the headunit side: can't get to ground through a cap! Even the integrated CK3000 bluetooth/carphone works great.

Thanks for listening. Once the dash-kit arrives to make the whole thing less frankenstein-ish, I'll post some pictures.

-brendan