1995 F150 Fender mount relay solenoid
Posting this up for anyone needing to see the wiring.
As you can see There are 2 red wires, a big one and a small one.
The big one comes from the battery and feeds the solenoid and also all those other ones attached to it at the right side bolt. All those extra wires at that spot take power to the other truck systems.
2 large red power wires come from the battery - one goes directly down to the solenoid on the starter, the other one is the large red wire seen in this pic.
The small red wire that attaches to the left post gets power when the solenoid is activated and directs power down to the solenoid on the starter, activating it and letting the power that's already there (coming straight from the battery) turn the starter.
The fender mount solenoid is activated when power comes to it through that little wire with the push on connector (at the top centre). That power comes from the ignition switch in your column when you turn the key to start.
I'm not 100% sure how far back this style goes but for sure 92 to 96 but I'm not sure of the change over year.
Prior to this system, full battery power used to go through the fender mount solenoid and out the other side and down to the starter. That put a lot of juice through the fender mount solenoid so the later style (picture) was developed so that full power went straight to a solenoid on the starter and the solenoid on the fender was used as a relay.