up to what year do the doors fit on gm cars

Instead of looking it up for you I'll explain it so you can do it in the future. Get on Google and look up "89 buick regal (part)" in your case part would be door lock. Go to one of the web pages that pops up and get the part number for the part that fits your car. On that same page search for that part number and 99% of the time it will come up with the results and an entire list of what it fits on. If you need any help reply here.

By the way it generally only works for "mechanics" parts and not so much on "body shop" parts.

The 88-96 are the same body style. You can also try Chevrolet Lumina, Chevrolet Monte Carlo, Oldsmobile Cutlass Supreme & Pontiac Grand Prix of the same years. They are all built on the GM "W" platform. You might be able to get the locking mechanisms from the scrap yard instead of replacing the entire door.