What should I expect to find above garage ceiling (drywall)?
I re-did my garage ceiling a couple years ago. Home built 2003, in Southeastern Ontario, Canada, so may not be entirely the same, but probably fairly close. Here's a picture after I had taken down some of the drywall, which gives a nice profile shot:




The drywall was mounted on another set of joists (I think they were 6") hung below the actual joists for the second floor, with about 18" from the bottom of the floor joists to bottom of the garage ceiling joists. 
There are a couple I-beams holding up the second floor
There was batt insulation laid on top of the drywall
Drywall edges were sealed with caulk to keep gasses from getting into the house (though someone prior to me had played hockey in the garage, and there were a few puck-shaped holes in the drywall)
No vapor barrier was installed 
HVAC and return vents run through this space 
There was a dedicated 6" HVAC vent blowing into this space, and a hole cut into one of the existing returns about 6' away to act as a return. I guess this was to heat the space to make the upper floor feel not as cold, but it wasn't overly effective and I'm sure it was a waste of money.
The wiring for most of the garage lights/plugs ran through this space; I don't recall any other wiring though.




You'll probably note I used the past tense there. I took all the drywall and insulation down, cut the joists back to make the ceiling higher, installed some network/cable jacks in the rooms above (I fished wires between the joists to get to rooms not over the garage), installed central vac, and then used spray foam to insulate the ceiling, used the old batt insulation to insulate the exterior walls, and finally dry walled and installed some shelves on the ceiling. The room above is warmer, the garage stays above freezing, and both cars and the snow blower fit (though I have to move everything else up to the shelves in the winter).