Digital Matter is currently developing a G120 Iridium Harness for easy connection to the Iridium Edge.

The Iridium Edge has a male M12-8 connector. 

The G120 Iridium harness features a 24 way connector - for connection on the G120 side. 

The necessary wires needed to connect to the Edge are terminated with a female M12-8 connector. For easy plug and play operation. 

If you wish to make your own harness, you will need to source a female M12-8 connector. 

The connector on the Iridium edge is IP67 rated, so it is worth ensuring that the female for the G120 side is also IP67 rated to ensure the rating is maintained across the connection. 

Connections are as follows. 

[5] Power -> external power 9-32V. I would just leave a loose lead. 
[2] Ground -> pin 20 (ground) on G120 24 way harness
[3] Edge RX - > pin 22 (G120 TX) on 24 way harness
[4] Edge TX -> pin 21 (G120 RX) on 24 way harness
[6] On/Off -> pin 11 (Switched GND 1) on 24 way harness

Iridium Edge Pin
G120 Pin
5 (External Power)
Leave a loose wire for connection to a power source.
2 (Ground)
20 (Ground) - Black wire
Any ground on the G120 harness can be used as they are equivalent.
20 is convenient though.
3 (Edge RX)
22 (G120 TX)

4 (Edge TX)
21 (G120 TX)

6 (On/Off)
11 (Switched Ground 1)

NB - the Iridium Edge needs to be mounted with a clear view of the sky. It is worth creating a harness long enough to allow this. Typically 2m between the G120 and edge should be enough for most vehicles.