I’ve replaced the relay and bulbs for my '92 Rodeo but only the brights work when I pull my dimmer switch. How do I fix this and what is the cause of this??
Check the headlight switch with a voltmeter or ohmmeter.
Be more specific. Are you turning on the headlights or they automatic? Have they worked perfectly for 19 years and suddenly failed “out of the blue?” Some of the Isuzus used a combination of 3 relays to control the headlights and regardless of the cause for your problem an accurate wiring diagram would be needed to quickly track the problem down. And yes, a volt meter or even a test light along with the wiring diagram will be needed.