If you can get a copy of the Gregorys manual or the Max Ellery manual, thay have wiring diagrams that should point you in the right direction.
Getting the dash out isnt that hard, just time consuming cause of the amount if seperate bits of plastic you have to take off. Also, you can push the speedo cable back throught its gland in the fire wall a couple of inches to make it easier to unclip from the dash panel.