[HO455]

OK first thing to do is verify all the battery cables are clean and securely connected on both ends, positive and negative side. A bad connection at one of those four places could cause your symptoms. (I am assuming that your negative lead goes to a nice clean spot on the engine. If not it needs to.) Then with a test light or meter check the purple wire to a known good ground for 12vdc when the ignition switch is in the start position. The plug at the steering column with the two purple wires needs to have a jumper from one purple wire to the other one. The other plug is for your back up lights. The purple wire will continue through the firewall and to the small bolt usually marked S on the starter solenoid. If you have power at the switch then move to the jumper at the steering column. If that is proves good move to the large purple wire in the upper half of the firewall plug on the engine side. If that is good now you have to check for power at the solenoid. If you have power there the starter or solenoid is bad. There are lots of drawings online for 72 C10s which should help you. If you have no power at the switch then check the large red wire for power in both the run and start positions. Each time you move your ground lead to a new location check it against a known good source of power, or run a ground lead to the neg post on the battery that is long enough to reach everywhere.