Speedometer problem, resolved: another question!
I've got a 1993-94 Pontiac Sunbird that began having speedo problems -- the needle would bounce around, sometimes be correct, sometimes go to zero, sometimes go to the end. I took it to an indie shop, and the mechanic did what your girlfriends did -- I think. However, he replaced the speedo sensor with a new one, first thing -- but not much changed. The speedo ran properly for a few days, but then bounced a few times, and finally quit altogether.
So I brought it back for more work -- the shop says they've done all kinds of tests and can't locate the problem, except that there's no power at the sensor ... so this part of the problem sounds a lot like what you had with your Sonoma, but requiring a different solution.
At any rate, after $300 for the new sensor installation and tests, what I have is a speedo and odo and cruise that don't work at all.
What can go wrong with the magnet on the flywheel in the transmission? Can it fall off the wheel? become de-magnetized? Or is the problem more likely to be electrical elsewhere?
Also, do any of youse have a clue about the bouncing around stuff? I've seen the question on other car forums, but haven't seen any answers at all ...
Thanks -- Phil