(Partial) Success!

Pins 7 and 18 it is -- got a good CAN-B connection and am able to read the data. Now comes the really time-consuming part where I reverse-engineer what's happening on the bus.

I also got the flaky speaker situation sorted out. I was blaming a loose wire going into the back of the factory harness, but it was actually a pin in the connector that was bending out of the way when I connected my custom harness. Nudged the pin back into alignment and everything seems solid. *crosses fingers*

Whew. What a hassle. On the bright side, I've gotten really good at field-stripping my car's dash.