Your truck is quite a bit older than I am used to working on, but in general, assuming that the headlight switch is working properly, one thing you might want to recheck is your grounding of the cluster. If the gauges in the cluster are working properly the cluster housing is well grounded, but you could still have bad grounding at the bulb sockets themselves. Because all the bulbs are affected, I would suggest checking the cluster grounding first and if the lights are still dim after confirming a good ground, check all the the sockets individually.
