reverse light switch

If the 2 outer posts on the trans mounted switch don't show closed circuit in reverse position you probably don't have that rooster comb inside the trans.
You are speaking of shifter mounted reverse lamp switch while pics above are of the column mounted switch. Do they share common parts? Who knows?
To first buy the 69 model rooster comb, a trans filter service kit, and fluid is one option. Drop the valve body to change the rooster comb, maybe replace the shaft seals there too. Then you would need only to disconnect the console/extension harness below the steering column and route those 2 purple wires through the bulkhead and tape in with the brown neutral safety wire.