it looks like that model is not the "black box" one, but you would have a sonar gsd mounted somewhere. since you are saying that the error is radar, it is mostlikely a connection directly into the rear of the MFD, the radar itself, or a coupler or splitter if one is in line. for example, i mounted my radar power converter in the port electronic (radio) box and have a coupler for the garmin network along side it. that way, i only run a network cable down to my mfd in the electronics pod. that coupler went bad on me last year and i had to replace it.
i also mounted my gsd on the underside of the electronic pod top. if you can't see yours, there may be a network splitter (expander) somewhere that the radar is also connected to. i would look in the overhead electronic box for connections and also behind the instrument panel in front of the electronic pod (there is actually a lot of room in there).
they could have mounted something in the locker next to the head also, i have my heading sensor in there. long run for a connection though, but if you have an AP could be there. next would be behind the midberth starboard bulkhead. last would be under the liner of the electronic pod; that would be a pain.
i would follow the radar network cable from the pedestal down and make sure all connections are tight and clean (no green). good luck, ron
ps. just found some old garmin error codes i had. don't know if they are the same but 6 was a power code. if so, check the power connections from the converter to the pedestal and in to the converter itself from the distribution panel. could also be a blown fuse? i did have a converter go bad early on, seems they had a bad run at that time. good luck, ron