Upvote:4
Nope. Essentially, you have to fly all legs of a trip.
IF you miss a leg, intentionally or otherwise, the airline assumes you can't make the other legs, and cancels all of them.
So while you may physically be able to drive to LA in time, you'd have been deemed to have missed the first leg, and there'll be no seat for you to Cancun.
It's known as hidden city ticketing, and we have quite a few other questions on it in different combinations, for example - Do you have to take the second leg of a flight?