As was the case on my 99, the placement of the factory fogs did not meet the state code. Therefore the bulbs were removed and the function was programmed out (they would not work if the bulbs were installed). The dealer was adamant about doing it. I finally convinced their body shop that I was installing their brush bar and lights, and they hooked the computer up and activated the system.
There are two things to be activated, the swirch, and the indicator on the dash. If you want to install aux lights rather than putting in bulbs, there is a connector behind or around the washer tank with a cap on it, the MB harness plugs into it.
They did charge me $50, the later ML lamps are compliant, other states allowed the previous.