Are you asking where to find the cruise control circuit ?
I can tell you to find it in the wiring diagrams in the tech section, on the 81-87 instrument pg1, top of page in the middle. It list 1 - 6, 4 being cruise control.
It appears to be on the back of the instrument cluster.
But you are going to have to wait till someone can tell you where to find the wire you are looking for and what to tie into like you asked.