65 coronet.
I redid the tail light wiring harness a while back, and it worked fine. I replaced each socket with one with a ground wire in addition to the other wires.

Recently I pulled the gas tank and electric fuel pump. Put all that back together. That is the only work I have done.

The passenger side stopped blinking. The dash light just stays solid on.
The driver side works like normal.

I checked and the tail light was blown on that filiment. So I replaced it with a used one, that looked good. So now it lights up.

Would the switch cause only one side to blink, and not the other?

When a light bulb is burnt out the signal light for that side will stay on and that side will not blink. It creates an open circuit.

Indeed, reduced load on one side (one or more bulbs burnt out) can cause that side not to blink.

To split hairs, what is going on is too little load. The turn signal flasher is merely a low-capacity circuit breaker that heats up from the load and opens the circuit. Once open, it cools down, and closes once again. This is what causes the blinking off and on. Without enough load to heat it up enough, it never opens heats up and opens in the first place.

I will try a new bulb and see if that fixes it. Thanks.

Check the front too. I know how your luck works.

Check you didnt put the bulb in backwards also.Its easy to do.Rocky