Side mirrors fuse constantly blows

I have replaced the fuse for the side mirrors several times (fuse #6 , engine bay fuse box) and I am scratching my head at what the problem could be. They work fine for a few miles, but after starting the engine the next day, the fuse goes again. They seem to work fine without the engine running (having replaced the fuse first!). I appreciate that it is a short circuit, but I was wondering if this is a common problem and someone knows the best place to start looking. Cheers all.