Like said above....you can try swapping relays. If the lights still do not work, my money is on the BOO switch. I worked at Mercedes for 4 years and replaced dozens of them on other chassis with the same part number switch.
In a perfect world, you would have a multimeter or at least a test light and I could walk you through a few steps to diag. Main thing is to see if you have voltage coming into the BOO switch and voltage out when the plunger is depressed. If you have voltage out, then next would be to check the relay.