I had a similar problem, and just fixed my broken switch (at least temporarily). My switch could be disassembled by prying apart the case of it once it was removed; there are a couple of melted plastic rivet pieces that hold the case together. Once that's done, it's pretty straightforward to disassemble and clean the contact pieces.
The two external contacts form hooks internally, and the rotating plunger piece has a short round bar/wire through it that swings up into the hooks to complete the circuit. In my switch, at least, the problem was crud and corrosion on these contacting parts. Scrubbing them in soap and water and then burnishing them with baking soda cleaned them up nicely. The switch unit seemed to hold together even with the rivet bits not properly upset, though if it starts coming apart, I think I can secure it with superglue or plastic-friendly epoxy or something similar.
Time will tell how permanent a fix this is. Given the cost and aggravation of looking up and ordering a replacement switch, I'm willing to give it a try. If it doesn't last well, I think I'll swap my light over to an LED and rig up a magnet and a reed switch to turn it on and off.. They'll have to be a transistor or something to do the actual switching, both because reed switches have low current ratings and because they are usually normally open, and close when the magnet is nearby—so a magnet on the door would cause the light to turn on when the door is closed without a transistor or something to invert the logic.
