1993 Ford F150, 336,000 miles and runs good. Recently found passager headlight out having changed both lamps after 14 years of service. Replaced both lamps and driver's side high & low beam works correctly. Passager side low beam will only work if I turn on the high beam first and then switch to low beam.
If I have the switch in low beam, only the driver side head light comes on. If I switch to high beam, both headlights come on and then after about a minute or so I switch to low beam and then the passager side low beam will be on and stay on as long as I like.
What can be causing this?

 

A defective bulb shorted internally would be my first guess. Swap the bulbs side to side and see if it follows.

Adrianspeeder