OK, I took the column apart, and unmounted the multifunction switch.
You can actually unclip the plastic overtop of the wire terminals going
into the switch and use a tester or multimeter to test. I see the
flashing power on a pair of terminals when the turn signal lever is
clicked, so I assume those wires are also the wires that go hot when
the brake switch is pressed. Also, on the white wire, I can see power
there when I depress the brake pedal, but nowhere do I see outgoing
power to the tail lights. I am pretty sure this switch is the cause of
GM dealer quoted me $380 for the multifunction switch!!! At
monsterautoparts.com it's $260, and at gmpartscheap.com it's $248.
Quite a spread so far... so I checked locally - napa wants $236,
Advance Auto Parts wants $175, and at Pep Boys it's $150. This does
look like a more expensive part - it's got the turn signal arm w/cruise
and wiper connections attached to it, as well as the hazard light
switch is built into it.... but I think that $380 is way out of line.
I asked each store plenty of questions, and some seem like they know
what I'm talking about, some of them seem like they have never seen a
car before. Some of them ask me if I have cruise, and tilt steering
(the dealer did, and told me there was a difference) and some of them
don't bother to ask and when I prompt them they tell me there's just
the one switch available.
Looks like I'll have to take mine around with me to these places and
compare so that I get the right part.