Great!!
You could actually reverse the yellow and dark wire. The relay coil does not know the difference electrically. I don't understand your test results. Either you did not have the meter grounded? or meter function switch set wrong, etc
The "functional path is........transmission ground (in park/ N) through switch...........to center pin.........to dark wire.......up to one relay coil terminal............through coil...........to yellow start wire...........through bulkhead connector-------into interior to ignition switch connector--------through switch "in start"--------to 12V switch supply
The relay contacts are the remaining two terminals. One of them, "the big stud" is also used as a junction point. The "square" terminal then feeds power (relay closed)----------down to starter solenoid.