Hi all,
I have a strange problem. my left side brake light doesn't appear to be working properly.

If I have my headlights on and I press the brake the left tail light dims and the brake light doesn't come on.

Also, If I have my front wipers on and press the break or indicate, my rear wiper comes on by itself?

any ideas?

Classic symptoms of a bad earth.
Could be on the light cluster itself or the lead to a body earth.

The wiper issue may or may not be related, I'd check though all earth thoroughly and for chafed cables.

Or if a new car, faulty light unit, but the same thing as Murphyv310 has pointed to.