qpr_clk

Ok guys so I have a 52 plate clk 240. I love the car to bits but there is an outstanding issue which is really getting to me!! Basically, I was driving along when the brake light malfunction came on after I stopped off at a petrol station. So I went back inside and bought a 21W bulb to replace the bulb. I pulled rear light electrical plug from the bulb mount and then managed to pull the entire bulb unit out. I replaced the bulb in question and reconnected the lot. Switched on the engine and still brake light malfunction showing and the bulb doesn't even work. I tried again and thought maybe it was just a dud bulb but the same situation. What is going on ?? Do i need to buy a mercedes benz bulb? In all my other cars this is minor trivial problem, however i just cannot sort it out in the clk. Please help!