Looking for 1974 Plymouth Duster wiring diagram

Surrey as in South of London. Yes?
The quick way will be the same as I suggested above, except for 1973.
It will be in the 1973 Dodge Chassis Service Manual.
Service Manuals – MyMopar

The 1968 Master Tech Conference explains how to work the wiring diagrams used through 1973.
Browse MTSC by Model Year – 1968 – MyMopar

There are several other electric related booklets in 1973 you may find helpful.

A good complete index of almost all of the Master Technician Service Conference is here:
Master Technician Service Conference - Chrysler's Training for Mechanics
Unfortunately the Imperial Club is having web issues and many of the links are dead. So use this as a table of contents and then go to Chrysler Master Tech Service Library – Browse Model Year – MyMopar and view it there.

The 1970 Hamtramck Registry Library Page (1970 - 1974)
has most of the Technical Service Bulletins (updates/corrections)

To buy a copy in paper or on CD, try Bishko. They're reputable and will ship overseas.