Does anyone have instructions as to how to replace the opera window molding/trim on a 1974 Cutlass Supreme

 

Yours is a one-piece plastic molding with a corner clip ('73's were a two piece stainless with interlocking ends). Simply use a flat-bladed screwdriver and tap on the outer edges of the molding towards the inside to dislodge it from the sheet metal perimeter of the window opening. Since it's weathered and brittle, don't need to use a lot of care in removing it. Install of new repro is just the opposite, of course. Just use care when tapping it into place (use a small block of wood...or, some such thing to prevent hitting directly on the new plastic). Ideally, it would be nice to find a used stainless set from a '73.