What is wrong if even after changing the brake light bulb on a 2003 VW Gti MK4 it is still dim and not visible in the daytime?

What is wrong if even after changing the brake light bulb on a 2003 VW Gti MK4 it is still dim and not visible in the daytime?
Make sure the correct lamp is installed. Make sure the correct filament is lighting.
Check voltage at the socket with the lamp installed. Probably will find the socket has a bad ground.