Brake Light Fuse Problem

[iandy]

Hi,
I've been having a problem with the brake lights of my 1999 Polo CL.
The brake light stopped coming on, i checked the brake light switch (the plunger thing) myself, which wasnt broken, and is working as it should.. checked the fuse, and it was the fuse that had blown.

Now, when i press the brake switch, the fuse blows instantly, but not every time, sometimes it will last a day or two before the brake light fuse pops. Any ideas why this is happening? I've checked the brake bulbs and everything appears to be in working order, anyone know if this is a common problem and any way to fix it?