yellow bypass relay?

Looks sort of like an old Ford style starter relay. Mine is mounted at the top of the firewall drivers side, 1997 379.

I have my eye on a Delco-Remy 39MT which has the magnetic switch or relay on the starter. Instructions say if your truck has mag switch/relay on firewall to continue using it in the circuit with the starter mounted one as well. I did find a Delco-Remy video that stated you could delete the Peterbilt mag switch/relay but would have to re-route the wires down to the starter relay which is the way I would do it. No sense in having two relays and two points of potential failure.

Throw a voltmeter on the starter positive and negative while cranking, if it's under 10 volts there is an issue in the wiring or battery.