O.k I sorted it out. Here is what you need to do ...
You need a Kenwood cable with part number E30-3374-05. By the looks of it (after much reading and searching), this Kenwood cable comes with the Kenwood TH-D7's, TH-D72's and TM-D710's (It could be that this cable comes with other Kenwood radios, but I cannot say for sure). It has 3 wires, RED, WHITE and a bare (stranded) copper wire. It has a standard 2.5mm jack on one side and no plug/jack on the other side.
The Garmin Serial/Data cable part number is 010-11131-00. Just connect them as shown in the picture.