headlight issue in 2006 A4 cab

Hi - I just bought my audi in July and there is an issue with the headlight the dealership can't seem to figure out - i've already taken it in 3
times and have another appointment for next week.
My left headlight does not come on all the time. It seems that when the weather is cooler(ie 90 or below - i live in az, in the summer that is cool :-) ) this occurs, and then after 20 min of driving the light will come on.
Last week they replaced the headlight unit, but that did not fix it.
Any ideas? Any experience with this?
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