You can check the Moparts Tech Archive. Lots of information on the subject, plus you can Google search about the topic as well.

Oftentimes, the Headlight Switch in our old cars is an antique and rather corroded, a weak link. Replacement switches are available and relatively cheap.

Beyond that, rewiring using relays so as to not send the heavy headlight current through the headlight switch and firewall connection, is a way to get rid of the dimming-at-idle and overall brightness issues.

Upgrading the wiring to the alternator ( along with circuit protection, fuses, fuseable-link, etc) can help the entire system as well.