Hello, I will try help you with this.
Have you or anybody else recently replaced any of the rear bulbs?
Is it the brake light comes on whenever you switch on sidelights or the sidelights come on when you press the brake?
both whichever way you do it they allcome on together
That would indicate the fault is with the brake circuit.
You need to recheck the bulbs are the correct ones. What you can also do is remove the brake bulbs and then press the brake. If the sidelamps still come on then there is a short circuit in the wiring between the brake circuit and the sidelamp circuit.