Some ideas:

There should be a ground connection near the left kick panel E brake pedal area. You will have to be on the drivers floor looking up to see it, 10MM head w/black wires and a star washer. Make sure it is tight.

Look for damaged wires under the carpet where the drivers left foot rests. Check the flasher push button. If it is in partially it can cause many problems, just push it in and pull it back out.

Look for a bad connection in the trunk on the left side where the tail/brake lamp harness plugs in to the harness going forward to the passenger compartment

Lastly try another t/signal switch, just plug it in at the bottom of the column as a test before completely installing it