the front right head lamp electronics does not get voltage -- needs replacement

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Hi The front right head lamp of my 2000 Audi A8 does not get voltage. Apparently I am told there are two parts to delivering voltage to the light. 1. The base 2. The balast One or both are out. How can these be tested? how much does the procedure cost? The Item number is I J 0941641 DS HID Balast -- if it is indeed the problem. John