[rudeney]

It is part of the fuel sender unit, which is on top of the fuel tank, under the rear seat on the left (USA driver's) side. Unless there is an aftermarket solution, the pressure sensor is not available as a separate part - you have to but the entire sender unit. And then you may have problems disconnecting its in-tank fuel lines from the the pump unit (on the right side) because the plastics get brittle and break. So often times, you send up having to replace both the sender and pump units.

 

[rudeney]

Sometimes you get fuel leaks around the seals and they can collect road grime. Also, oil leaking from the engine or transmission can makes it way onto the top of the fuel tank dues to air currents flowing under the car.

As I said earlier, the pressure sensor is not a separately available part, at least not from MBZ.