There are some posts by, I think Tony H on how this is done. I took his advice earlier this year for my 91 245. Take your stock speakers and cut out the entire back end, leaving just the flat rim that goes on the door panel and the angled part that takes the speaker grill. Now you can mount any 4" speaker you want to the plate. Removing it to begin with means cutting the rivets, no big deal. I replaced it using stainless nuts and bolts in the original holes. OEM covers snap right on. I used Jensen 4" coaxial speakers I got on E-bay. They are shallow, but you are below the window so space isn't that much of a problem. I have actually saved some old blown speakers so I can do this again when the situation arises on another car.

DS