the solenoid should be automatically grounded by being bolted to an engine bracket, this is the Ford system where the starter solenoid is not on the starter motor itself.
did you make sure that when you turn the key to START, that you have 12V between the yellow/red wire terminal and ground? Because if the solenoid has current and you have 12 V at the yellow/red terminal the starter should engage and crank over
PS this is a Ford Starter, so it might be a bit different than the more common GM design. Color codes should be the same though, they are universal. Checking my OMC shop manual it looks like the Ford starter is mounted separate from the starter, the solenoid has one heavy gauge wire going from it to the starter and at the solenoid there are 4 terminals, one for 3 heavy red cables, one heavy red cable to starter, one for yellow red (S terminal) and one for the black/blue terminal (I terminal) . Of the 3 red cables on the one terminal one is from the battery or batt switch, one is from the pos terminal on the alt (could be an orange wire after a fuse holder) and One goes to the down solenoid for the trim system (red turns to a blue/red after a fuse holder). Still should work the same, the one terminal with 3 red cables should have battery volts if your batt switch is on, and if you connect your remote starter switch to this terminal and the yellow red and hit the button it should crank.
