Flashing Beacons Problem

Hi

Yesterday I put 2 Britax Mini lightbars onto the back of my transit connect. I connected an Earth to the body of the van and ran it down the earth spade on my switch and ran the live down the the live spade on the switch. ran the other live wire from the switch into the wire of the interior light, and i put a 15amp fuse in that line.
The Beacons work fine BUT when they are off the interior lights are very very dim, but when i switch the beacons on the interior goes back to normal brightness!
Any idea what ive done wrong?

Thanks in advance