Changing out the voltage regulator on a '91 300SEL is without a doubt one of the EASIEST tasks I've ever performed. If you have it in your hand and the car is already up on ramps, it might take you 3-4 mins. altogether. 2 phillips head screws come out and so does the volt. reg. The new one will have brushes about 1/2' long, so you will have to cant them a bit to get them in. No big deal. Even though you have no elect. connections to undo on the vol. reg.(at least I didn't), you might want to pull the battery grnd. strap first just for safety sake.