Originally Posted by sixto

Does your car have reclining rear seats? If so, my SDL had those but I don't remember how they come off, which they have to to replace the rear shocks. If not, there's a tab at each end of the seat cushion where it meets the carpet. Slide the tab towards the door while lifting the forward edge of the seat cushion. Pull off the seat cushion and set aside. There are 3 screws that take an 8mm socket holding the seat back - one at each lower corner, one under the center armrest. With the screws out, lift the seat back about an inch to clear the hangers along the shelf then pull the seat back forward and free.

If you're mounting the brake light along the lower edge of the windows, you might be able to run the wires under the shelf liner towards either door so you can tap into the brake light wires that run under the door jambs then under the seat cushion before going into the trunk. I don't remember how the shelf liner comes off and I think it depends on the speaker arrangement. You might have to remove the sail panels which will be brittle and delicate at this age.

Sixto
83 300SD can be yours
98 E320 wagon