To start, I made a base out of foam board. A base made out of wood would have been sturdier, but for a throw-away base, the foam board works well enough. I covered it with cake foil to make it food safe. After all, who wants bits of plastic foam in their cake? I used actual measurements from the ambulance and adjusted them a bit to make the cake wider and to evenly match up with the pans I have so there would be as little cake waste as possible. I just eyeballed the wheel locations because I forgot to measure that. I elevated the cake board by cutting some foam board pieces just smaller than the board. They were taped together and wrapped in black posterboard to make it less visible. The board has to be elevated to allow for the wheels to look like they are holding the ambulance up. The problem with this setup is that it makes the cake a bit unstable. It's especially true for a top-heavy one like this. Here is a picture of the board with the cab part before I carved it.
To make the wheels, I make some thin Rice Krispy treats, cut them round, and covered them in black fondant. Then I painted silver hubs. They are held on with toothpicks. The side-view mirrors are gum paste molded over tiny biscuit sticks. I was going to use bamboo skewers, but I found these biscuit sticks in the japanese food section at Wegman's. They are thicker than bamboo skewers and tastier. This allowed me to use one less non-edible item. It bothers me whenever I have to put something non-edible in or on my cakes.
Here is a photo of the final cake. I think most of the shifting may have been from all the moving around I did. The foam board base has some give to it and it's not like the foam board was 100% stable anyway. It was sitting on that narrower riser. I really must figure some way around that.