Alot of folks do this because of a failed fuel computer.

I am not sure what model you have so I will tell you how I did it for my 89 240DL.

Mine has a White fuel pump relay located under the right front kick panel.

I installed another relay to switch ground to the connector on the Fuel Pump relay.

This ground source normally comes from the crank sensor or the fuel computer and senses that the engine is rotating; hence the fuel pump will run.

You must install another relay. Connect a switched 12-volt source to pin 85 on the "New relay" and a good ground to pin 86 and 30. Then connect a wire to provide the ground when the relay recieves a power source to pin 87 on the new relay. This wire will need to be connected to 86/2 on the Fuel pump relay.

You should be able to test this before you install the relay with a test light or a ground source, just turn the key on and touch the ground source to the 86/2 pin on the Fuel Pump relay...You should hear the fuel pump humming. Then just install a relay to provide the ground you need.