What motor do you have on your X?

Although removing the bumper cover is not that hard, there are a couple of things you can do to diagnose without removing it.

First thing I would do is swap igniters and bulbs side to side.

I asked which motor because I've done the above without pulling the bumper cover, on my facelift 3.0. Not sure if it can be done on a V8.

If the problem doesnt move, than I would swap over the ballasts, which will require pulling the bumper cover.

I doubt (and hope) you dont have to replace a whole headlamp assy.

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04 X5 3.0i auto
03 X5 4.6is