First make sure that all fuses are ok
I presume that when you switch off the light switch you lose the lights
If so then you need to start checking the supply from the brake light switch
A voltmeter would be really helpful
if no voltmeter is available then how about a small test bulb
one side of the test bulb setup to the negative side of the battery
then go testing to see where you have power on the brake light circuit
if none of this makes sense then you need to start looking for someone who has a bit more knowledge of vehicle electrics
no offence meant