There's 2 wires that run from the ign sw. to the ballast, one of them provides full 12V during cranking only, the other is the run only power wire, they a brown and dk. blue. You run power wire is the problem. find it and youre gloden. Could be switch....

Take care,
Rick
68 Coronet R/T 440 & 68 Charger 528 Hemi,and 5 Challengers! 6 cyl, 318, 360, 383, 451