Your car has one fuel pump. It's located at the rear, just to the left of the right rear tire.
The pump and filter and housed in a plastic shroud held on by 3 bolts. I believe a 10mm socket gets these bolts off. Afterwards you have access to this area.
It helps to back the car up on ramps to allow more room to crawl underneath if you're are going to DIY this yourself.
1991 300-SEL - Model 126
M103 - SOHC