Hello,
You were right ... it was the radio wiring. I did a very careful check of all the circuits with a test light and jumped power from one area to another to bypass circuits and isolate the issue. The second white/pink wire to the radio from the original radio wiring was the culprit. Apparently, it completes a loop and energizes the wire which then runs to the NSS. If you can explain it in more correct terms that would be great ... trying to educate myself. As always. appreciate all the good info. from everyone.
The wiring schematics will be your best friend when troubleshooting these kind of issues. Here is the one for your car:
http://www.7173mustangs.com/images/1973_full.pdf
Look on sheet 5 at coordinates A-26. C-501 is the radio connector and you see the 2 wires to it. You can see that power comes from fuse F1 to connector C-501 to power the radio and then goes to connector C-211 and exits the page on the left to page 4 of the schematics.
On page 4 it goes to C-406 which is the connector for the NSS in the engine compartment. It goes to the center contacts of the NSS switch which is made when in reverse and then goes to the reverse lights through some more connectors. You can see the lights at the bottom right on page 4.
When you mentioned the radio change that was what I needed to narrow where the issue was.
There is a better set of schematics which I use that shows the connectors and describes where to find them as well as describing each circuit function and all the wires color codes. It is a good investment.
