W212 E class GPS antenna connector location

[AW10]

I'm trying to help a neighbour with a 2011 E350 solve a nav system problem where from time to time the location of the car is miles out. This will last for some random number of days and then the problem will go away and the nav system is back on track. I'd like to check the connection between the GPS antenna and the rest of the car. Any idea where the connection is? The repalcement antenna at https://www.commandonline.co.uk/W204-W212-W207-Sil... looks to have a lead of 1-2 metres which suggests the connection is in the boot somewhere. Any pointers before we pull out all of the lining would be appreciated.