My defendant agrees to purchase a car at a dealership. He signs the loan application and appears to sign the purchase contract.

After the defendant leaves the dealership with the car, the bank tells the dealership that the defendant did not sign the purchase contract -- instead of signing he wrote some gibberish. The dealership calls the defendant and asks him to return the car or sign the contract. Defendant refuses and it takes several weeks to repossess the car. The dealership does not attempt to cash the down payment check and the car is retrieved before any payments are missed.

The jury is not given any instructions on theft by deception. The Defendant was convicted of theft and UUMV.

So my question: is this a crime or a civil contract dispute? I'm looking for ideas on how to defend these convictions.

BTW, I believe that under Texas contract law where one party accepts the benefits of a contract his signature is not required for a written contract to exist. On the other hand, the contract here expressly required a signature from both parties.

Is it a civil issue? Yes. But, that does not preclude it also being a criminal issue.
Where's the defendant's case law that says the two are mutually exclusive?

False Promise of Future Performance: A fraud claim based on a false promise of future performance is actionable as false representation. Haase v. Glazner, 62 SW3d 795, 799 (Tex. 2001). To prove a false promise of future performance you must establish the defendant made a promise with no intention of performing it. Spoljaric v. Percival Tours, Inc., 708 SW2d 432, 434 (Tex. 1986).

Keep in mind that a party commits fraud when it enters into a contract that it does not intend to perfrom; a party merely breaches a contract when it enters into a contract and later decides not to perform it. F.S. New Prods. v. Strong Indus., 129 SW3d 606, 619-20 (Tex.App. - Houston [1st Dist.] 2004).

Deceptive conduct is the equivalent to a false statement of fact. So when the guy did a 'fake' signature, he made a false statement of fact that this fake signature was his 'real' signature.

If the offer specifies the manner of acceptance (i.e. the contract here expressly required a signature from both parties), then the offeree (the guy) must strictly comply with those terms. An acceptance is effective if the manner in which it is made strictly complies with the terms of the offer. See Town of Lindsay v. Cooke Cty. Elec. Coop. Ass'n, 502 SW2d 117, 118 (Tex. 1973); Cantue v. Central Educ. Agency, 884 SW2d 565, 567-568 (Tex.App. - Austin 1994, no writ).

Hope this helps!

The issue, I guess, is whether the defendant had the car dealership's effective consent.

Was it deception to not sign the contract with the defendant's name when the dealership clearly knew who the defendant was?

Is theft-by-deception -- see PC secs. 31.01(3)(A), 31.01(1)(A), (B) -- part of a hypothetically correct jury charge?