Something is probably shorting to ground. If so, when you put in a new fuse, it will blow, too. The easiest way to troubleshoot this type of problem is to wire a 12-volt light bulb across the fuse contacts (without the fuse installed). You need a test light, or a spare bulb and some alligator clip wires to do this.
If there is a short, the bulb will light up, but the circuit will not be damaged, because the bulb will limit current. Then you disconnect related connections until you get the light to go out -- that means whatever you did removed the short. Keep going downstream in the circuit in the same manner until you isolate the problem.