Need help wiring in dash reverse indicator light

This is an automatic car converted to stick, or what?

"3 blades?" Where? Plug into what? block?

It sounds to me like you might have a harness for a 68 or 69 or later automatic, IE three round pins. Four speeds have two flat blades

Electrically this lamp runs from the backup lamps line to ground. The backup lights wire runs through the tail harness connector in the left kick panel

Do you have? a factory shop manual?

Download one at MyMopar for free