Has anyone replaced a damaged rear quarter panel on a 1995 w124 E300D?

Those are extremely difficult compared to front fenders.

Whereas a front fender just bolts on, the rear quarters on a unibody are usually stamped from one large piece of metal comprising the area from the a-pillars, to the roof seam, down the back window, to the rear bumper and back along the rocker sills to the drivers door hinge area.

The panel is joined to its neighbors by welded joints.

I'm not sure how the pros fix it, I suspect they cut out the damaged area and weld in a replacement piece.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

"Modern" car design uses a one piece side, older stuff is built up from many pieces. Have a look on a factory parts manual to be sure.

On older cars ( or when repairing newer stuff,) the quarter is cut off part way up the C pillar. ( this is usually a rear window out job. ) Modern repairs use structural adhesive where possible rather than welding as it is faster, eliminates warpage and provides good sealing against rust.

Adhesive does allow long seam repairs opening up the possibility of running along a body line rather than a factory seam. ( think van quarter panel replacement where factory seam is at roof level )