Registration and Posting Agreement
So neither the uppers nor lowers light with highbeam. Sounds like it might be a dimmer switch or connections at the dimmer. Unplug a headlamp and check for power (cause it's easy to get to). As I recall, black is ground, tan is low, and green is high. If green is dead, check the dimmer. If green and tan aren't dead, check the ground.
So I have noticed that dash lights have been flickering and I have to adjust the dimmer, but the low beams work fine. So I'm confused as to why it would affect the high beams when they work off the wiper/cruise switch on the column.
I had the same thing basically with my 86 . Make sure the connectors at the lights look good not broken or melted . If you do have to have new switches, I recommend using Delco replacements, the cheaper ones i bought first were not as good and the cheap headlight switch i bought had a faulty circuit breaker inside it (non-servicable). You have good advice in checking all grounds and connections. Good luck and keep after it!