How do you replace the automatic transmission shift knob on a 1980 450 SL?

You're going to need a Philips screwdriver and I believe a 13 mm open end wrench, and a 10mm socket/ratchet with a 6" extension. Park the car, use the parking brake, then move the shifter to low. Remove the ashtray and then remove the ashtray frame...I believe there are 2 Philip's screws holding the unit in place. Manipulate the frame out of the center console. You can disconnect the ashtray frame's electrical connection. Remove the two Philip's screws on the front of the center console . While gently pulling up on that wooden cover, carefully pull the unit back. It won't move too much because of the electrical connections (window switches and hazard switch). Once that is removed, there are 2 or 3 10mm bolts holding down the black plastic shifter box. Remove the light fixture on the left side of the unit. From there you can manipulate the black plastic box up. You'll see a black rubber boot,, which you may have to move. You should see the 13mm nut on the base of the shifter shaft. Loosen that nut. Once loosened, unscrew the shifter from it's housing. Once that's removed, you can insert the replacement shifter. Make sure you line the shifter up straight before locking it back down...then reverse the steps to button it back up. I might suggest cleaning up all that crap floating around there...and,, because its such a pain to replace the shifter housing light bulb...you might want to invest in the $1.00 for a new bulb. * Be careful when screwing the wood center console down...just snug, do not over tighten those screws or you will crack the wood. This is a simple job to do, and shouldn't take more than 30 mins for a weekend mechanic...Just be careful.