Our 1988 5th avenue does this: on high beam both the high and low beams
are on on the driver's side along with the high beam on the passenger side,
but on low beam the lights on the driver's side don't come on at all while just
the low beam comes on on the passenger side. WHat could be the problem?
How to troubleshoot? How to fix?
This is a thirty year ole car and you have to expect that things will wear out. You have lights, so don't suspect bulbs or fuses. You have a wiring problem; possible causes are a corroded socket or a wire; hot or ground; maybe in the headlight switch, or dimmer switch. Try spraying contact cleaner into the bulb sockets and cleaning any corrosion off the bulb. If this doesn't work, you should use a multi-meter to check current back to ground and to the fuse box.
the headlight switch might need to be replaced