I just replaced my speedo cable in my 87 300D and the only thing holding the cable to the tranny was a 10mm bolt! I don't know if this will be any help but the bolt on my tranny holds the cable to the tranny. It screws into the cast body of the tranny around the speedo cable port of the tranny and squeezes the end of the cable firm to the tranny speedo port. I saw no plastic cover you described.

..............BB