Thanks! That exploded diagram was just the info I needed. I'll add a couple of notes for the next person trying to do this:
There are actually two screws under each truck. The screws closest to each end of the car can be left alone. They secure the plastic inserts with the seating. It's the other two screws, closest to the center of the trucks, that have to be removed in order to take off the top of the car.
Once the screws are removed, the top will not just lift off. There are two pairs of retainer latches molded into the top that have to be released as well. If you count from the front (where the doors are) the first set is a the back end of the third window and the second set is at the front end of the 6th window.
The way I got the top off my car without damaging the plastic was to remove the screws and pry the edge of the roof at the front end (by the doors) up with my fingernail and then slide a small standard screwdriver blade between the roof and the car just above the door. I then did the same on the other side with a second screwdriver. I then slowly eased the screwdrivers toward the center of the car until the latches released and the roof popped off. By going slow and easy I was able to avoid damaging the roof and left the latches intact as well.
Hope this helps.