Theft
The advice to hire better people is good, but screening out perfectly is not always possible.
I don't know your procedures, but if you can tighten down which employees are allowed to access a vehicle you might be able to better tie the complaints back to a certain employee. And, I hope you are getting such complaints immediately-- and not a few days later?
Suppose only one employee has been in the vehicle, and this is recorded by a camera covering a wide area of the parking lot. Once you get a single complaint, there is at least a warning issued, and if another complaint comes in there is a termination. And that's being nice. If you had strong enough evidence you could probably terminate on the first incident, of course.
I like the idea of doing a setup vehicle as well. Just be sure to mark the
coins or record the bill numbers in advance just in case you need to identify the exact money that was stolen later.