Are they fused? Often times a short will make the lights go dim but usually will blow a fuse fairly quickly. Sometimes this rarely can even occur inside of a bulb if it burns out the wrong way, I believe.

I bet they'd be fused though, so unless somebody stuck a tinfoil-special in there, this may not be relevant advice as it should blow the fuse in that case. Nonetheless, make sure the fuse is the right rating and that there's no extra load (like mods) on this line as that would also dim the lights down.

Using incandescents, then, I presume?