Did you look properly for the GPS receiver?
It's mounted under the plastic floor panel in between the spare wheel well and the rear seat.

Cable: black coax cable, around 4mm diameter, fitted with SMB-male connector one end and SMB-female connector the other end.

Nothing proprietary about it at all, it is industry-standard SMB-connectors and 50-ohm low-loss coax cable such as LMR100.
You won't find it at your local hobby store, but any good RF cable place that supplies the cellphone base-station industry can make one for you. You need the proper crimp connectors, so unless you work in the industry you won't have the tools to make the cable yourself.