Mine were doing this when I first got it. It turned out to be the plug right near the driver's side light had a bad connection. When I squeezed the two sides together tight it worked fine so I did that I taped it up real tight with electrical tape. First thing I would do is get a test light and see if there is juice going to the lights. They both get power from the same plug.....
04 Ram 2500,Green/Silver 305/555HO,48RE,3:73s,QC,LB,4x4,Roadwire Leather Seats,Laramie Interior,Infinity Sound,HE351 Turbo,Custom 4" Exhaust,VA C3.2,Westach gauges,Torque Tube,BeCool Fan+180 Stat,Fogzilla,TRM,BFG AT 305/65s,72,000 miles