oily fingers touching the bulbs upon install? wear a latex glove next time to DISPROVE this, instead of thinking you aren't doing it.
Check the rubber seal around where the lightbulb seals in the the housing.(orange or black rubber ring) check for water, even though you say there isn't any..
measure voltage at the harness with those lights turned on and see if there is a difference in voltages between driver's and passenger's side.
Ray
