Wiring question

Hi all,

so I recently fitted a new sound system into my classic Impreza. Annoyingly though I have somehow managed to stop my rear lights working. Brake lights are fine in fact all other lights work fine. Does anyone know if there is a connection behind the head unit I may have got wrong and if so what colour wire I should be looking for when I take it out again. I'm sure it won't take forever to work out but if I could get a quick answer that would speed things along. I've been meaning to write a post about the install but will wait until the boot build is totally finished. 

Ps it's not a fuse  

Thanks everyone