Looks like the left rear door underneath at the seam is problem area for rust.I recently replced a door on a 92 that was showing a lot of rust.For the heck of it I checked my 90 which is in very good condition and I noticed it to will need some work in the near future,will replace the entire door.I have looked at some in the yards and it is always the rear left that has the rusty seam on the bottom.I have also checked and none of these rusty doors had clogged drain holes.I wash and wax my cars often and the only thing that I can think to do is to PARK the car at an INCLINE for little while after washing so that doors actually drain.I think the fact that the car is always level is adding to the rust problem.Have you checked your seams lately?