My 74 450 sl had the same problem,the cable route was not very straight,kind of wavy,so any debris along the cable would slow up the return action. I very carefully removed the out side rubber like covering in several places about an inch or so long. Sprayed in a derust chemical and worked the cable back and forth many times ,repeating the chemical addition,It took a while for the chemical to run the lenght of the cable but after a couple of days the cable was free and returns back as it should.