back-up lights - '68 4spd barracuda

getting around to looking into these non-functioning lights - bulbs are good and sockets and grounding llooks good. Single purple wire to each - I suppose first check again to see if any power in the wires when in reverse with ign.sw. on? I don't think I have ever messed with this circuit on any car so any troubleshooting tips appreciated. I did replace the turn signal switch in the steering wheel and rewired all as it was last year. I don't think the back ups were working before that.