Gmc sonoma voltage fluxuation & headlights dim at night

Bulbs do dim over time but they shouldn't be 'that' dim.

With the engine running. Remove a bulb and use a test meter to test the voltage at the bulb socket, it should be between 12.5 and 13.2 volts.

The with the engine off, test the voltage at the battery terminals. It should be just over 12 volts.

Then test the bulbs with 2 wires to the battery.

Spray the bulb sockets with WD40.

Are the back reflector surface of the headlights in good condition?

Have you polished off any yellowing and dirt from the headlight plastic lenses?

Have you checked the voltage at the headlights ?
If the high beam work ok, it could be a problem with the dimmer switch.

First, check for output voltage at the headlight switch when the switch is turned on. If you do not get output voltage at the switch, replace the switch, and you'll be good. Otherwise, read on.
A common problem with those GMC and Chevy trucks is that they used too small of a gauge for the headlight wiring under the dashboard. You will need to use a voltmeter to check all points of the headlight wiring from the switch to the headlights to determine where the voltage is lost, and will very likely need to replace some wiring. PLEASE BE SURE YOU USE HEAVIER GAUGE WIRE THAN WAS USED BY THE FACTORY, so that you can avoid having to re-do the work in the future.

Go to a parts store and have them test load your battery and alternator normaly they do it for free.

Replace the voltage regulator

11-12 IS GOOD WITH RUNNING NOTHING ON WHATS IT ON . NOW TURN JUST HEAD LIGTHS ON DID IT MOVE ANY MAYBE 1-2 METERS THATS FINE . DRIVE IT!!!!!!!!!!!!!!

Could be bad voltage regulator in alternator.

one of the gauges must a fluxuated enough to have the computer turn the light on,I've seen mostly gas gauges fluxuate,but train your eye where the voltage is in normal mode because it will still look like the needle is up there but it may only be 11 volts instead of 13-14 volts running.

voltage regulator replace it

Check your battery connections and do a voltage test to battery with engine running lights off, then check with lights on. You need a minimum of 12.5 volts but should have 13-14 volts even with lights.