There is a one on ebay.de, but it is a Westfalia, and it only fits S4 and later 928s : http://www.ebay.de/itm/Porsche-928-S...item53e8805aa3

Found an old thread on the trailer hitch coupling.

I have what seems like the one-and-only 928 Factory fitted with the trailer hitch....

I will try to take some pics of the mounting when I have the rear bumper cover off for painting soon. :-)

Pics of the factory mounted tow bar. Will take more pics when I have the bumper cover off.

https://cimg8.ibsrv.net/gimg/rennlis...9c68ecf13f.jpg
https://cimg9.ibsrv.net/gimg/rennlis...25f674832d.jpg
https://cimg0.ibsrv.net/gimg/rennlis...9154f8dad6.jpg