Rapid left flashers when braking but not with 4-way

2001 base Taco. Here's my fault condition: Left signals flash rapidly, brake-light dims and flashes and my (not OEM) rear cctv monitor cuts out, all only when brakes are applied. No connections between lights and cctv. All return to normal when brakes off. Left side only. Does not occur with 4-way flashers. When in fault, switching on 4-way returns all to normal, 4-way off, back to fault. Headlights make no difference.
There's a trailer light adaptor in the taillight compartment, which crowds things but not horribly.
Some months ago I installed trailer wiring but everything worked fine until now.
First time I took the tail-light off, didn't find anything obvious, buttoned it up and fault gone. Signal worked fine for about 20 trys, driving on a bumpy road, then fault came back.
Replaced all bulbs, same result without road bumps.
Went in again, removed two now-unused wire taps, taping over cut insulation. Same thing.
Signal switch?
Thanks