18 is the wire guage and T is Tan. Itīs not the wire for the dimmer switch.
It is just not logical the way the wire is. Is starts on the firewall and ends on the firewall and is not part of any curcuit.
@ Drew - Nice collection you got there! Thanks for hosting the diagrams for and from Francisco!
Here are the direct download links from Drew:
Master Plan
Engine & Front Lights
Rear Lighting
Bulkhead #1 (Cab)
Bulkhead #2 (Cab)