I did some measuring this morning on the front door speaker. It looks like many 5 1/4" round speakers will go in that position. The distance across the face of the speaker and the grill opening, (not counting the thick rubber isolation pad surrounding the speaker and meant to contact the removable grill) is about 5 1/2". The depth from the door panel to the back side of the speaker grill varies from about 1 1/2" back by the armrest, to 3/4" up at the bottom front of the door. The depth of the basket that holds the speaker varies from ~1 1/4" to 1 1/2".
"Basket that holds the speaker"
This means that the thickest (deepest top mounting depth) 5 1/4" or 5x7" speaker that should fit in there is about 2" 'top mounting depth' specification. It would seem that a spacer ring of some sort will be needed between the 'basket' and the speaker to allow the depth clearance, and that the least complicated install would be a 5 1/4" round speaker. as I see it at this point, the speaker would mount to home made spacer ring mounted on the 'basket', and the whole assembly should mount back in the original speaker position. A ring of dense foam (from the original speaker?) would be needed to isolate the speaker face from the rear of the speaker. Also, the tweeter in the center should not have a lot of protrusion above the surface of the woofer if it is to clear the grill.
I have not tried this install yet, and all the numbers are approximate, as are the installation details. For example, if there is clearance behind the speaker 'basket' and the windows/locks/outer sheet metal don't interfere, the back could be cut out of the 'basket' to gain some additional mounting depth (a waterproof shield would be needed to deflect away from the speaker back). Also, 2" is a worst case number - measured at the forward edge of the door. Towards the center or armrest end, there may be more clearance. These details will probably be modified as I proceed, but this may be a useful starting point.
On my 98 C230 the first aid kit is on the top of the rear shelf and the subs are on each side, to the rear of the headrest. just for my own info, where is the amp located, and how is it accessed? Thanks for any advice.