You probably have a poor/corroded ground on that side. The extra resistance in the circuit reduces the voltage to the bulb. Electrical people call this “dropping” voltage.
Find the ground wire to the lighting harness and be sure it’s clean and well secured. You can probably get a wiring diagram (different from a schematic, it’ll show you where the wires physically run) from the dealer’s parts window for free upon request.
Or, the connector might be dirty. That’ll drop voltage too. Get some “contact cleaner” from radio Shack and flush the connector out.