325i brake light problems

[ra28gt]

I am having problems with my brake lights, I have changed the brake light switch already so basically when i turn the ignition key to number one before the dash lights come on my brake lights work, as soon as i turn the key to ignition two this is when i have all the dash lights on the lights do not work, i have also put a multimeter on the brake light feed i get 12v but it drops to zero when i turn the ignition to number 2

Any idea what it could be, ?