And when I step on the brake left turn signal indicator comes on and radio light dims.

 

Buy a new one

 

Your description reads like a brake light bulb is using a turn signal bulb for its Ground, if possible have some one push down brake pedal, or use a stick, and see if a turn signal bulb is "glowing" when brake light are on, that would be the bulb being used as the brake light ground, so check the grounds on that taillight

Check if someone has added trailer wiring under rear bumper, could be a short there, over time water and debris from rear wheel and a bad wiring job will deteriorate causing problems

In 1997 the brake lights do not go thru turn signal switch, so shouldn't be a problem with that

 

I disconnected the bulb connector's on the left front and rear lights but the anomalies on the dash ( the left indicator arrow would light when I depressed the brake) persisted. I disconnected the right rear bulb connectors to no avail. With all bulbs connected I disconnected the feed to the trailer harness adapter and the turn signal arrow and front lights did not come on when I stepped on the brake. I have ordered a new trailer harness adapter.

 

Good diagnostics, nice work

 

New trailer model solved the problem

 

Thanks for the update and the FIX