The mice spawn inside the car and the shaman spawns on top of it. The mice will probably end up pushing it one way or another, possibly going back and forth between the edge of the map and the edge of the gap.
While trapped inside the car, the best you can do is make sure it doesn't go too far in either direction. However, it's very possible to escape the car due to lag. If that happens, you might be able to make the gap by air jumping, or just wait for the shaman to build something.
Your starting position is on top of the car. The simplest way to get it across the gap is to create a plank bridge, however lazy shamans might use a cannonball to knock the car across. If the mice don't outright panic, this method sometimes works.