This is an image of the bowden pressure cable arrangement on a 1991 300CE. Probably typical for any 300 during that time. Picture taken with the air cleaner removed. Bowden cable is attached to the throttle linkage and is directly behind the throttle body.

Using a 13mm wrench, turn the plastic yellow adjustment nut counter-clockwise when facing the vehicle to shift sooner, clockwise to shift later. The arrow and mark were not necessarily aligned when I had it where I wanted it. Be sure to inspect the vacuum line to the vacuum modulator for a tight fit while you're at it, but don't tug too hard or you will pull it loose from the transmission end and you will have to crawl under the car to reattach it.

Please note: This car is new to me and I still tinkering with it and learning it, but I thought that this might be a start to show you what at least one bowden cable arrangement might look like.