Multiple electrical glitches in older Fords are often caused by the piece of crap ignition switches they used at the time. The switch consists of two halves that are held together by metal tabs bent over a plastic frame. The switch overheats so much that the plastic melts and the pieces begin to separate. It's possible that pulling on the highbeam dimmer flexes the wiring harness enough that it temporarily reconnects the switch.
Remove the steering column cover. The ignition switch should be on the underside, about 6-8 inches down from the key cylinder (they're connected by a linkage). If it looks or smells burned, that's almost definitely the culprit.
