You have a short somewhere would be my first guess. Possibly something else pulling too much power from that fuse.
A bad solution would be to put a slighly larger fuse in there.
A good solution would be to find what's pulling more Amps then is allotted for that circuit.
Possible you hooked something up there, or a wire is exposed. Very tricky problem sometimes.
Have you installed something recently that pulls power? Some aftermarket guage, a radio, foglights?