OK - So I am not sure that the W220 uses the same solution as the C215, so...
The GPS antenna in the C215 is located underneath the boot lid which is made of fiber glass. There is a second joint in the CL inside the boot but you need to remove the inner textiles in order to gain access to it. I experienced occational loss of sattellites and later no connection at all.
The GPS antenna is powered by +5V at the center of the coax cable that runs to it.
If you disconnect the connector from the antenna you should be able to measure and check for this voltage. Use a standrad Volt-meter. Do not short circuit the center pin to the outer shell.
Depending upon which type of Comand you have, there is a possibility to enter an "Engineering" mode and from there check for error codes. This is enabled by pressing a number of keys simultaneously for a few seconds. Google and thau shalt find.
In my case, it was as simple as a bad connection. Wiggleing the coax connectors back and forth and the problem was gone.
Hi I will try and give it a go tomorrow or Friday as I'm off for a few days. Anyone know which is the best aftermarket GPS that will give good guidance and will fit to the existing system and maybe just sit on the rear parcel shelf. Sorry if I seem to be coming on too strong but just excited about getting a Mercedes and want to get it right.
Just plug it into the connector block that normally resides behind the panelling on the left-hand side of the boot, replacing the old one.
You can just lay it on the rear parcel shelf to ensure it works before hiding it under the cloth part of the shelf.