The car doesn't get power to the fuel pump all the time.

I'm sorry I can't help diagnose it from here, but there is something else missing from the equation if you truly power the fuel pump manually and can't start the vehicle. Probably it is something faulty in the hypothesis of how the fuel should be provided to the engine. I would suggest having a diagnostic test by an experienced Mercedes-Benz technician that has resources of diagnostic equipment and manuals.