Thanks for the reply, but that isn't for the Custom Cruise III system, which added the Accelerate function, which was introduced on the 83 trucks. The one in the link just has resume and is only a 3 wire harness and is for the Cruise Master system. The 83 has a 4 wire harness and a different plug.
The only picture I can find of it is below (first one of two in attachment). if looks almost identical to the one the previous reply suggested, but if you look closely, it has a R/A (resume/accelerate) option, instead of only Resume.
The second link says it will not work with OEM systems.
Also, I previously purchased the one linked above that I would sell to a member in need for $75 + shipping. It is NOS in the original plastic, GM #25030993. Fits an 81-82 truck, and maybe some other GM vehicles of that vintage.