Backup lights work with turn signal

[68convStang]

Hi All,
I reposted this because I think it was in the wrong forum.
I have 68 conv. New turn signal switch was added to fix horn. Fixed horn but had issues getting drivers directional to work.
I got that fixed. Had to put in neutral safety switch and back up lights because po swapped out tranny and didn't put them back in.
Original transmission was a c4 current one is fmx. New neutral safety switch is for fmx tranny. Now backup lights work fine but when
I turn on right directional the backup lights flash at same time. It does not happen on left side. I ran ground wires separately from
just the backup lights to eliminate problem in brake light harness. Is there anywhere on the lower valance that could be affecting the ground ?
I've thought of checking for a bad ground but not sure where to start. New member.
Thanks.

 

[7991LXnSHO]

this is wild, but I’ll brainstorm with you.
If it is a ground path that is causing the back ups to blink with the a turn signal, I would suspect the tail light/TS bulb/housing to not be grounded right. But as these are on separate switches and are at least mostly on separate circuits, I’d be looking for a wire diagram to see what has been messed up by a PO. Could the power for the back up lamps be taken from the left TS power wire?