you can buy a new one, or try spraying a little silicon spray or a small quantity of some other lubricant, on the seal ring on the cap. (light blue ring on mine).
The engine light / fuel cap warning is coming on because there is a leak in the EVAP system. That is probably caused by the fuel cap.
The most common cause of an EVAP leak, and hence pollution, is an incorrectly sealing fuel cap, that's why Mercedes give you the red warning because it is the best, simplest, bet. (It could still be something else.)
Soo, the red Fuel Cap warning light is back, and it set of the Check Engine Light.
Guess I wasted $44.00 on a new fuel cap. Called my indy mech, he is going to check MB service bulletins for more info on the problem, still tempted to try silicon grease on the fuel cap seal: If that fixes it, it would be a "no-cost" item.