tail light oddity after on frame bed restore

[hplacecf]

hi.

1977 chevrolet c10 stepside.

did an on frame restore of the bed.
took off the steps, fenders, tail gate and front box panel, removed and replaced bolts and nuts.
shifted stuff around to get rid of rust, and then paint.

starting to put it all together.

started with the tail lights. turned on the lights and the tail lights worked fine.

but, if I turn on the left blinker, the left blinker turns off and on completely (even with tail lights on - it goes completely off), and the right side backup light appears to blink in time with the left blinker. ?!?!?!?!?

not sure what is going on.

then, I happened to move the bed slightly and the tail lights went off completely.

I moved the bed again and they came on.

any ideas?

maybe a ground wire?

I searched the forum and did not find anything. I do not see any loose wires.

thanks