Good Method to Add Wiring For Add-Ons
From somewhere I have a partial digitized '74 electical section. Might have been from FABO a few years back.
@mopardude62 analysis looks correct to me.
Pull the hand brake with the key on, the brake light lamp should light up.
Wire codes:
J2 is Ignition. So key in run, it is hot
G was probably Gages
P maybe was Parking Brake originally