You don't unlock it.
The Used Car Dealership operates on a stock rotation basis. For the most part this is pretty simple: cars appear in stock in a predetermined order, stay in stock for a certain amount of days, then become "Limited Stock" on their second-last day, "Out of Stock" two days later and greyed out, and then disappear for a certain amount of days as they're replaced by the next car on the predetemined list.
Sometimes there's curveballs. A car may be absent for a lot longer than usual, or it may appear out-of-sequence as a "Special Pick" - usually with a much lower mileage and in a rare or unusual colour.
However it's all controlled remotely and there's nothing you can do about it.
When it was first added to the game, the Escort RS Cosworth appeared in stock for two weeks (which is usual for newly added cars) - November 24 to December 8 - before dropping into the regular stock rotation. That should mean it's back in the second week of January.