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From what you've told me, it sounds like the dealership should reinstate their discount since the original contract is back in force. As they are failing or refusing to do so, it seems as though they may be in breach of contract.
The first step you may wish to take would be to send the dealership a demand letter. A demand letter is a letter generally written by a lawyer (since that makes it more 'intimidating' than you personally contacting them) demanding that they pay you back the excess money you've paid and/or reinstate the discount, failing which you will commence legal action against them.
If this demand letter does not work, or you wish to forego it entirely, then you could sue the company in court. If the amount is less than $25,000 (which seems to be the case), you can sue them in small claims court which is a very streamlined and simplified way to litigate. Alternatively, you could file a consumer complaint and try to arbitrate the matter that way. This site may be of assistance for this latter option: http://www.consumerscouncil.com/index.cfm?id=18244
Hopefully that answers your question.