I had this problem on my '93. The assembly was so dirty that WD-40 didn't help at all. I ended up buying new switches from the dealer. The switches are push pin style switches mechanically similar to the push switch that activates your interior lights when you open the door; they're mounted in the latch assembly of the door. When the door shuts, the bottom of the door latch pushes down on the pin switch. They're cheap, and they weren't terribly hard to install.
The switches aren't built into the latch assembly, they're attached to them on the bottom of the assembly and are interchangable. Just remove your door trim, reach in the door, twist off the old one, and twist on the new one and plug it in.
Look at the wires leading to the door latch to find it.
Bear in mind that there is an additional pin switch on the assemby that activates your interior lights when you try to open your locked door from outside the car, so be mindful not to waste your time going after the wrong switch when you get to work.
The only difficulty I had was having a good view of what I was doing since there isn't much light in there. If you're not affraid of removing your door trim to work on the door, new switches are the way to go.