Jboggs:
What yr. is your 560SL?
If it's a 1900 or 1991, it may be equipped with what's called a MAS. Silver box on drivers side near fire wall. It has a knob on top that you loosen counter clockwise. Once loosened, the whole unit comes out.
Ports 1 & 2 are jumpered to power fuel pump(s). These 2 ports are what's used by the fuel pump relay. The FPR is incorporated into the MAS on cars equipped with such.
If you jumper 1 & 2 and the car starts, the fuel pump relay within the MAS is bad and the MAS has to be replaced. If not, your pump(s) may be bad. The MAS is $500 + at the dealer.
I once read here that the MAS can be opened and soldered. I'm not an electrics expert, but would wonder just how a diyer would go about opening this unit?
Be careful when you jumper 1 & 2. One or both is wired hot and if you don't hit them both just right with the jumper wire, you'll get a spark.
Understand that you really cannot operate the car like this. Jumpering 1 & 2 will prove if the FPR is functional or not. If the car starts and runs jumpered, you'll get a Check Engine light. This will reset itself once the MAS is reinstalled.
MAS units that contain Fuel Pump Relay
If your MB does not have a MAS, locate the fuel pump relay and jumper the correct pins. I seem to recall they are 30 & 87, but a SEARCH would verify this. If you have a faulty "independent" FPR, your wallet won't get hit as hard.
If the weather holds up, I'll be replacing my MBs fuel pump this weekend.