If you are getting 12v for a second with the key switched on and then 8v with it cranking it sounds like the battery is dead.
You are supposed to read battery voltage at the pump when it is switched. See if the battery voltage drops similarly when cranking. it should be no lower than 9.6v when cranking and a good battery will stay well above 10v. There should be no more than a few tenths of a volt difference between battery voltage and switched pump voltage.
Bosch Master, ASE Master, L1
33 years MB technician