The FSO can be replaced without replacing the IP,it is at the frt of the IP and looks like a stack of 50 cent pieces with a couple of wires connected,it just screws out of the IP but can be very hard to get out if the O ring is stuck and often wrecks the FSO when you do try to remove it.
If you hold your hand on it and it clicks firmly when the key is turned to run,it is likely working.
Have you checked the fuel/sol fuse?
It powers the PMD,the fuel solenoid and the FSO.
The PMD being mounted on the fender is not ideal,having airflow while good when driving does not help when the the vehicle is shutdown and underhood temps skyrocket from heatsoak.
A sudden temp spike in any direction it what aggravates the internal connections of the PMD,over time the repeated temp swings work the connections and they fail.
Mounting the PMD combo outside the underhood area is best so that it stays at or near ambient temps at all times.
With a quality extension harness and the proper medium between the PMD and heatsink, moisture is of no concern.
Heath Diesel did a test with a PMD/heatsink/resistor combo immersed in a bucket of water and no problems were encountered.
Also while it is good to have all grounds in good shape,the IP harness ground must be fastened to the IP or the drivers side battery ONLY!
The length an IP can last is not something that can be measured,if the truck has run for an extended time with a non working LP-it can damage an IP very quickly.
Or if no lubricity additive is added to the fuel then the IP also wears more quickly.
Likewise some rebuiders re-use old head and rotors while others rebuild with new parts-both affect how long it lasts.