1) A few. Don't remember who and Street View is incomplete.
2) The garage was completed in 2003. It's possible that they're no longer in use, but 6 years isn't that long.
I'm 99% sure it's not AVLs for the busses. They wouldn't gain much from putting them way up high, and if anything it'd give them false positives due to the increased range. I couldn't figure out how to word this link in that last sentence, so I'll just put it here:
signpost system.
By "car" do you mean train car or transit vehicle (bus)? Metro doesn't send much besides location back to base. They did a pilot project where two buses did, but that's been discontinued but is planned to be integrated in the system wide radio upgrade. That's their whole 700 MHz project, which won't be completed until after 2011.
And Sound Transit's buses aren't any different, as they're ordered and built to Metro's specifications. Sound Transit's buses are operated by Community Transit, KC Metro, and Pierce Transit. The individual agencies do all the maintanence, storage, and operation (which is why you'll occasionally see a Metro coach doing a ST route or vice versa) but ST pays for it all. So a ST bus operated by Metro has all the same radios and stuff.
And Sean won't say what they are because he can't (or he doesn't know).
I guess it could be an AVL-type system for the trains, but why they need 5 antennas is beyond me. And if it was for the trains I'd think they'd be a bit closer to the platform.