One thing, do they know or could they have figured out that you use these forums? Ignoring the fact of what's being discussed in this thread, I think you posted what wage you would be earning when you got offered the job, maybe they saw that and if it's higher, then that is why the bullying has started? Also a lot (and before I get shot down; NOT all) of fitters/garages seem to have this culture of offensive banter, spraying each other with grease guns, hiding tools, disabling cars, swearing, (not so)jolly japes etc... It's not an excuse, I'm just saying. The likelihood is that the wiper thing was just supposed to be a prank and they did not intend to damage your car, it does not make it right, just saying.
There is also the fact that people that have been in a job a long time without any career progression don't work as hard as a fresh faces newb trying to make an impression, so that probably worries them.
Best of luck here, keep notes as already said, don't do anything back, don't do any pranks, keep it professional and work to your standard not theirs!
Cheers
Stu