Yeah... I'm betting the pump is on a circuit which is not powered in the START position then.
Try leaving the key in the On position until you hear the pump stop running because the rail is fully pressurized, instead of sticking the key in and slamming it straight from OFF to START. Then crank the motor over as briefly as possible to get the key back to the ON position the MOMENT the motor is spinning good, so the pump kicks back in. A buddy did this to his rig, execpt the aftermarket *COIL* was in a ciruit like this, so he only had spark in the RUN/ON position, but not while in START. Once we figured it out, repeatedly bumping the motor over for about 2/3 of a second would get it some spark right as the motor spun to a stop, and it caught every time from then on, on the 3rd or fourth bump, until he got home from the trail ride and could re-wire it.
That was right after we stole one of his winches solenoids because the starter solenoid got fried by all the cranking, naturally.