Yea, the coil goes open in those relays from time to time. I find the best option is to order a new RCM, then get the old one fixed when I can. OVer here in the USA, I just send mine to DJ. He has a huge inventory of those relays, the secret is to by them in lots of over 100, they get cheap then.

I would not go to the trouble of a "kill switch", if you have the OEM SKREEM/ECU system from the factory, there is no need to add anything to it. One thing you can do, in order to move the car, is to wrap a rubber band around the relay, holding it closed, so the car will start. But you must remove that rubber band once you move the car and turn the engine off.

For an RCM., it is often less expensive to search for: "2003 Mercedes SLK320 Relay Control Module" than for a Crossfire part. IT's the same part, but many warehouses do not realize it's the same part.