low beam headlights not working (sometimes). High beams work fine. Mechanic troubleshot and couldn’t find anything wrong, although the low beams weren’t working at that time. He suggested replacing control unit but wouldn’t guarantee that would fix it. Could it be a grounding issue? If so, where do I start?
Find another mechanic. Do not throw parts at a problem hoping to fix it. You will have the same results if you toss dollar bills into the radiator fan when it is running, money spent with nothing to show for it. Wiring problems on an older car can take a while to track down. Corrosion, age and wear and tear can cause a bunch of odd hard to find problems. But they can be found and fixed or worked around.
If you do not know where to start looking for a grounding problem you are already in over your head. But if you want to charge forward, buy an inexpensive voltmeter, Lowes, Homedepot, Harborfreight and others have them for less than $30.00. Check for proper voltage at the bulb socket (12 vdc) . To check for proper ground, find the ground/return for the light and make sure there is continuity to the nearest metal part ( body/frame, etc) Sometimes you will find a corroded connection that needs cleaned up. Sometimes you need to add a new wire to bypass a corroded mess. If you live in an area that salts roads heavily there may be a bunch of rust causing you problems. Not fun, lots of tedious checking, but it can be done.
I think the low and high beams share the same ground and if so the ground would seem to be okay in that case. I would first suspect the light switch as the cause of the trouble. If the lights are DRL type then that circuit may be involved also.