Reverse indicator light

[Mad Cuda]

I have a 73 Barracuda with an older 383 4 speed. First I can't get the reverse lights to work. Bulbs are good, wires going to front are good. I noticed the backup light switch wire harness badly frayed and not connected. Im looking for a new wire harness but can't find one for a 70-73 manual tranny. I may have to repair the one I have. Question: The small dash cluster that has the headlight switch contains a small red sense that I have no idea what its for. Is possibly a reverse light indicator or something else? I know that earlier models have a reverse light indicator on the dash, next to the ash tray. Any idea what that red light lenses is for? Thanks