Attachment to brake light

I'd like to add an extra brake light at the back of my 89 205, for safety. You know the kind you buy in a shop, and then stick on your window, like modern cars have.

Being a particularly bad driver, I thought it wise...

But when I came to attach it to the brake light at the back, like the instructions said, I found that all the wiring was encased in the back light box, so I couldn't get at it.

Can anyone suggest a way to hook this extra brake light up..

thanks

Thanks I'll try that...

But there must be lots of wires.. how do I tell which one's the brake light...