While I like the sedan bumper replacement idea be aware that the wagon bumper is pretty low so if you install a light under it there's a good chance it won't last long. Here where we have major hills and driveways that are anything but flat it's not unusual to crunch the backup lens even with the stock setup. I think MBZ realized this was a problem which is why the lights were incorporated in the brake light cluster on the later 124s.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold