This is from the wiring diagram for an 88-89 300SE/L:

15 -- +12V from fuse 7, hot in run and start
50 (60?) -- start/cold start (signal?)
81K -- transmission kickdown
31 -- ground
81V -- cold start valve
30 -- +12V from battery
TD -- to AC control relay and CIS control unit from ignition control unit
TF -- CIS control unit
87 -- fuel pump/s and O2 sensor heater

It seems to me that you should be able to start, albeit difficult when cold, and run by jumping 30 and 87. If you don't hear the pumps humming when you jump 30 and 87, your pumps or the wiring to the pumps are bad.

Sixto
91 300SE
87 300SDL
81 300SD