This is an install of OEM 3 point seat belts in a 1985 Carrera. The car had lap belts installed, but had the factory mounting points on the rear shelf.
After some research, I decided to go with OEM retractors, which are a 964 part number (purchased from Porsche).
My approach was to make a template of the upper part of the rear shelf. I worked out that the retractors would fit under the rear shelf once installed and hence had to exit from the front face of the shelf.
NOTE- when making this panel, it is very good practice to line it up with the OEM panel and punch holes in the mounting screw locations. So when you test fit it to the car, you can use screws to attach it to get the correct vertical position.
The retractors were mounted on the rear shelf after trimming the insulation so that the units sat flat but could also be rotated to get the belt angle correct. The bolt thread was 7/16 UNF - the bolts I got from a seat belt installer (as these did not come with the retractors). The colts were about 25 mm so I used a washer and crush washer to get the right fit.
The rosette that I fitted for the belt opening was the same part the front seat belts exit from. This seemed suitable for the rear panel as others have done.
The approach I used was to test fit the retractors and measure the trace of the belt coming through the front panel. This is needed to know where to cut the actual trim. I used an approach to move the rosettes in a slot and then tape them in position.
I found that the retractors need to be mounted an an angle, ie not firing straight out, but at an angle such that the belt extends just to the inner edge of the rear seat back bolster. Also, the position of the belt in the rosette changes are you pull the belt, so it is a matter of moving the rosette so it has the best overall position.
I measured the distance from the edge of the cardboard panel to the edge of the positioned rosette (from LH or RH edge, 65mm, and from the bottom, 50 mm).
These measurements were then transferred to the actual trim panel using masking tape to define the position. I traced inside the rosette to define an initial position to cut
The upper panel is a very stiff, rubber/plastic material. To cut this, I drilled 4 holes, one in each corner of the initial square cut-out, and then using a very sharp knife (razor blade) I cut from corner to corner and then trimmed until the rosette fitted.
Here are some pictures of the finished result. I was quite happy with the position of the rosettes and the install looks the way I intended it to - clean and as "original" as possible.
