Thanks for the post.

I have a '93 W124 coupe, and decided to bite the bullet and upgrade the (dreadful) stock speakers. After prising them out, I cut away the "excess" foam around the dash's speaker holes until they were opened out to the bare metal. Each circle is around 114.5mm in diameter.

The whole thing is a bit fraught because the coupe has a steeper windshield angle than the sedan/wagon, meaning there's even less space to work in. With judicious razor blade carving of the foam immediately around the apertures, I managed to shoehorn in a couple of Focus ISC130, which are 5¼" coaxials. There's more space on the passenger side, but I was still unable to drill any holes for mounting screws so wound up using butyl rubber sealant to cement them in place on both sides. On the driver's side I was obliged to remove one speaker lug and cut away some of the black plastic stiffener framework on the underside of the speaker grill to make room for the tweeter. I was unable to smooth the butyl sealant very evenly on either side and it looks a bit rough, but that's all hidden once the grills are back on.

At first I thought the speakers were rather tinny sounding, but after several hours use at moderate volume they seem to be "deepening" quite well. They certainly sound a LOT better than the originals.

I'm currently installing a Kenwood KSC-SW11 subwoofer under the first-aid flap. This is turning out to be a little more complicated than anticipated so I'll post with my observations when (if!) I finish.