Please keep in mind, this forum is limited to providing you with "general information" so I can provide you with the general legal principles that apply to these kinds of situations, but I cannot tell you whether or not the facts and circumstances of your particular transaction would entitle you to rescind your purchase.
-Regarding diminished capacity: in general, reduced mental capacity of one party does not make the contract void or voidable. However, if the party is incapacitated in such a way that the other party knew, or should have known, that the incapacitated party was incapable of comprehending the nature of the transaction and its consequences, then the contract may become voidable. This generally only applies to individuals suffering from mental impairments due to medical conditions or similar situations, it does not apply to individuals that have caused themselves to become incapacitated by overconsumption, etc.
-Regarding misrepresentation on the documents: this is problematic for two reasons, on the one hand, you are arguing that you were too incapacitated to be able to fully comprehend what was going on, but at the same time you are arguing that you know that the dealer made a misrepresentation (your credibility as your own witness is at issue). The second reason is that if you are arguing that the dealer sent in fraudulent loan documents, you would have had to have signed them (so you signed documents that had improper information), the law has a presumption that the documents that a person signed have been fully read and understood, furthermore almost all loan docs have specific clauses to reinforce this effect in the signature block).
-You can attempt to renegotiate your situation with the dealer - they may not want to get into a dispute with a buyer over whether or not they took advantage of an intoxicated buyer, but the legal position that you have identified here is not terribly strong for getting you out of the contract.