On my truck when I try to put the headlamps on.

Suspect the head light switch. First look for overheated terminals in the connector and signs that the connector melted. If you see that, replace the switch, and cut out the blackened terminals and replace them separately along with the 4" of wire that will be hardened. If the connector and terminals appear to be okay, suspect pitted and arced contacts on the switches internal circuit breaker. Use a test light or voltmeter to measure the 12 volts on the feed wire to the switch. If it stays steady while the lights are flickering, test the wire where the 12 volts is coming out and going to the dimmer switch. If the voltage is fluctuating there, replace the switch.