This late model GT6 MKIII has three switches on the gear box. The normal reverse, under the gear lever extension, and two mounted on an adapter plate in front of the gear lever top plate. The two on the adapter plate are the O/D interlock and seatbelt warning light indicator.  From right to left in the above photo, the tunnel entrance connections from the main harness, the adapter plate with the O/D and seat belt switches, a ground return lug for the gear shift lever switch, a bolt down harness mount ear, the two terminals for the reverse switch, two connectors for the gear shift lever switch, and the single wire down to the solenoid. 

The tunnel entrance end of the harness
 

G green
N   brown
G/LG green light green stripe
Y/G   yellow / green stripe
Y/P   yellow/ purple strip

New seatbelt and O/D switches and custom
harness test fitted to adapter plate

   The harness continues down to the adapter plate for the seatbelt and O/D switches. (seatbelt switch is nearest you in this photo) The eyelet (yellow jacketed) the ground return for the gearlever mounted O/D switch. The harness hanger is bolted to one of the gear lever extension bolts. The last two spade lugs go to the reverse switch. 

O/D solenoid with protective wire cover

   The O/D had never been run or hooked up by the P.O. Now I know why, The solenoid wire was badly mangled. So I had to pull it out, clean it up, solder on a new wire and seal it all back up. I added and 'overcasing' made from a piece of tubing to protect the lead and route it up to the gear lever extension.  

New harness all in place

    Here's the new harness all installed with proper bullet connectors all crimped on. The feed wire down to the solenoid should be Y/P, but I ran out, so a 16 gauge red wire comes out of the harness leading to the solenoid. The terminals of the replacement reverse switch had to be bent over to clearance the gear lever extension.

O/D relay at left, horn relay on the right , flasher above

   Up on the fire wall, I mounted the O/D relay right on the same bracket as the horn relay.   The color coding was followed right from the schematic. When the battery shelf was being rehab'ed, I wrapped the new wires right into the main harness to keep it all original looking. I'll route the wires from inside the passenger compartment out the firewall hole along with  the original reverse and seatbelt wiring harness.

  Well here's a schematic version of the harness. It shows the routing through the two firewall grommets as well. The relay is routed through the main harness and through the fusebox grommet. I then ran the O/D wires out the lower firewall grommet along with the stock reverse and seat belt switch harness wires.