WTF - "don't like to"....as though you "like to" pay out of your earnings. It really boils down to if you need that job and reference. If you chase this down with the insurance company then sure the insurance company "wants" to know about all accidents. They want to know for themselves but care little about you as a person. Since the deductible isn't going to cover it the insurance company isn't on the hook to pay. The dealership then is "on the hook" to pay so to say. However you've waived that by entering a agreement with the dealership as a condition of employment to compensate them for damages such as this that you incur while employed. The dealership manager is under a tough economy and most likely is thinking "cut expenses" and workers are waiting on line for jobs he feels. You not only signed at employment, but then further signed (contracted) to pay all expenses. Before you signed you may have been able to negotiate a 50/50 deal with the dealership (unlikely it seems though in reality).
The law limits how much can be deducted from you per pay period. A similiar scenario is a cashier comes up $100 short on her/his drawer at the end of the day. They will recount that shit several times yet come up short. They will have to pay that out of their own pocket - I've seen this before.
In my opinion if you rustle too many feathers it won't be to your benefit. You might take on a free consultation with a lawyer but the lawyer needs to make money either from you or the "case" which doesn't appear lucrative.
Here's the federal stance on this for you to read
http://www.dol.gov/esa/whd/regs/compliance/whdfs16.pdf
You see how the minimum wage comes into play (you must make minimum wage after the deduction). However some states like Texas don't allow them to pull from your wages. What they would do if you didn't sign is fire you and then take you to court and get a judgement against you.
So the circle comes back. Check your state laws ! But with 2 signed contracts in hand the dealership is well armed to defend themselves.
IMO, you're best bet is to take responsibility and not begrudge your employer. Your manager didn't want to have to do it to you either.
Lesson learned to be careful what you sign in your future. You have a God given right to unlimited contracting protected by the Constitution for all legal transactions.
Good luck.