This post might be of no help for you (sorry), but is the dealer not able to carry out the process for you? We've purchased a vehicle here from someone who does this professionally and I had absolutely nothing to do with the traspaso process, even though I'm pretty familiar of how it goes. I just paid him to do it (reasonable amount based on my research). He had a copy of my ID and the original matricula and a week later he handed me the new matricula with my name on it. As the name of this other thread by WW on this forum, I called that "the easy life"...
Can't give you all the steps, but it's a little bit of a hassle, especially if you don't speak Spanish. Any dealer I've known will always do it for you. I bought a car from a private seller and paid an attorney to do it - I think about 4000 pesos, but that doesn't include transfer taxes and some other fees that have to be paid also.
Usually when you buy at a dealer the trapaso can be included in the price of the car or charged as an extra. The dealer will take care of everything and hand over the paperwork when he has the procedure finished. If it isnt done by the dealer you should get a lawyer to do it for you and make life simpler for yourself.
The process for purchasing a vehicle is:
1. Negotiate a price
2. Lawyer makes a contract seller and the buyer has to sign the contract, and sign the back of the matricula. Also when the buyer pays for the vehicle.
3. Lawyer takes the contract to be notarized and authorized by the Procuraderia
4 Seller takes the vehicle to Plan Piloto for inspection and receives a certified doc authorizing the sale. VIP service is available.
5. Seller and buyer take the authorized contract, the Plan Piloto certification to DGII to pay the tax and issue a new "matricula"
I (Meaning my lawyer) have done this process in as little as a day, never more than two. It depends on the lines at the different offices.
As others have indicated, If you're buying from a dealer they should complete the process for you. They usually take longer to deliver the matricula because they don't send a messenger to DGII every time they sell a vehicle- they usually accumulate them until they have a batch of them.