I replaced my original, non-retractable shoulder belts on my 69 280 SL with retractable belts from an early 80's 380SL. The retracting mechanism is bolted vertically under the parcel shelf and the belt then threads through the space between the shelf and the side of the convertible top compartment. The pivot mounts at the original shoulder point as does the floor mount. The piece into which the buckle snaps into must me mounted backward so that it doesn't impinge either the wooden center storage tray nor the seat's side. This backwards mounting doesn't effect function. I found the belts for $75 a pair from a wreck and I found that the only hard part of the installation was placement of the mounting bolts through the floor pan in the area under the parcel shelf as there are two layers of metal in that area. The installation is very clean looking, very functional and can be reversed easily back to the original belts with the only evidence being some holes inside of the parcel area where no one ever looks anyway. The retractable setup avoinds loosing the belts under the seat, catching the belts in the door and are more comfortable when you move around in the driver's seat (eg. leaning out of the window to enter the code to a parking lot.)
1969 280SL, manual, MB TEX, green/cognac
