Why does Fuse on Ford Focus keeps Blowing?

[flowingfaster]

The cigarette lighter fuse on my 2002 Ford Focus keeps blowing and I am constantly having to replace it. It's annoying because I use it for my sat nav and also for a generator.
What could be causing the fuse to blow all the time?
It doesn't blow when anything is plugged in, it's usually the next time I come to use it and find it's blown.
It's the standard stereo that came with the vehicle.

 

[friedach]

fuse blows on overload. you either have a short or your device uses too much power. NB fuse should be at least 10A.

 

[michael2k_18]

there's a problem with that particular circuit on the loom, you'll need to get it looked at by an auto electrician

 

[chris]

Do you have a aftermarket Stereo? Is there anything wired into the Cigarette lighter circuit that shouldn't be there?