The 55.2 GMC schematic/wiring diagram in the maintenance manual shows a neutral safety switch and no relay. The Starting section mentions no relay and the whole manual is available on oldcarmanualproject.
I don't think anyone said "There's no high current involved".
The ignition switch, used on 55.2-59, does not see high current. It activates the solenoid electromagnet low current. The high current is then isolated to the battery cable/solenoid/starter motor connections as the plunger slams. (Big terminals and copper strip/disc). The solenoid does not need a relay to protect it. The ignition switch does not need anything to protect it. It's all good and has worked for 63.97 years.
Please be sure you have a ballast resistor installed and throw the relay in the lake....I mean recycle.
Lets get this show on the road. There is no big deal here. Truck had an unnecessary relay in the system that was turning on another relay. It was adding resistance and now does not work and is stopping the whole shebang.
All of our advise and suggestions are off if the truck is a hybrid, mixed up wiring, hob knob, flux relay, previous owner franken-ignition. If lightning aura appears on the truck's body surface upon starting, be prepared to go back to the future.......in pieces.
