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It seems like it may be the ballast. Try swapping bulbs and see if the problem follows the bulb or stays with the ballast. If it stays with the ballast then just replace the faulty ballast. If it follows the bulb then replace both bulbs.
I have an '03 mcs and i also experience the dimming. I noticed at idle when the electric fan kick on, the lights dim but i don't know if it is the same thing while driving.