As pointed out, your 300SEL fuel pump/filter are just to the left of the right rear tire as viewed from the rear of the car. There's a plastic shroud that surrounds the pump/filter. This area is accessed more easily if you back the car up some on ramps. Disconnect the ground strap from the battery before you begin work.
The service task you need is on the 126 CD-Rom. There are likely better ways to get to it; try this one:
- Click on drive that houses your CD
- Click on PROGRAM
- Click on ENGINE
- Click on 103
- Click on task 07-269
Task 07-269 covers fuel pump replacement. Tasks 07-268, 7-270; 07-280 covers items in the area of your fuel pump to include the filter, accumulator; rubber rings/buffers.
1991 300-SEL - Model 126
M103 - SOHC