I was told (from the village idiot "I give credit to Mr.K for this term") that in order for a vehicle to leave a dealership that vehicle "must" have the proper coverage (full coverage) in order to leave the dealers property. Weather are not that is a contract between the bank and the dealer, or state law, I don't know.
But OP should check with the finance company and see what is in the contract between the dealer and the finance company.
If the dealer were to make sure that the vehicle had the proper coverage before leaving its property and failed to do so, then maybe the finance company will reverse the loan. That will still leave the dealer with the 20% deposit and the trade in, but it will relieve OP of a huge hit on his credit rating if he decided to stop paying the loan.
If the finance company reversed the loan, then OP could sue the dealership under the term that it breached a contract with the finance company which in return left OP with a loss.

