CLUE 13 - SOLUTION
And the answer is:
SFO INTERNATIONAL TERMINAL
As indicated on the back of the puzzle, we were to go to the International Terminal and look for a red Tercel, license number 2GDL316. The tag on the key told us where to go in the parking lot: Section A, level 6, area C. (I'm making that up, but it was something like that.) I don't know where the car key went, perhaps GC took it back from us at the finish?
And when we got to the parking garage, we did indeed find the car, where we unlocked it and found our next clue, plus a box of donuts, some cans of Red Bull, and something which looked like a joint (but I don't know, it disappeared and none of my teammates seems to know where it went.)
GC told us later that they had intended to leave the clues in lockers at the international terminal (and we were meant to use our quarters from Clue 10, hence the note on the roll of quarters "Keep These With You"). But at the last moment they discovered that the lockers weren't working, so they pressed a borrowed car into service instead.