IT could be a ballast, bulb, or your fuse. Flickering usually means there is a lack of power to the ballast. Since it is a 1991 vehicle, you need to upgrade your light fuses in the vehicle to brand new fuses with 15amp per side. If it is only one fuse for both lights, then 30amp. Essentially, what happens is the metal of the fuse wears down over time to the point it chokes the power...the same if you power too small of a gauge power/ground wire to a high powered audio amplifier. The fuse does not have to be popped to be an issue. Hope this helps.