If it only blows the fuse after the remote voltage is applied, it probably has shorted output transistors or someone has gone in and turned the bias up too high.
Disconnect all speaker wires from the amp and disconnect the RCA jacks. Replace the fuse with a 15 amp fuse. If the fuse blows when you turn the head unit on, the amp is the problem. If the fuse doesn't blow, you need to check the wiring.
If the fuse blows and you want to troubleshoot the problem, disconnect the amp and remove the bottom cover. The output transistors are connected to the large green resistors.
Measure the resistance from the middle leg to the third leg of each of the output transistors, If you find any group (3 transistors per group) that appears shorted, at least one of the group is shorted.
You can generally determine the shorted one by measuring the resistance from the first leg to the middle leg. The one with the lowest resistance is the shorted one. If you cut it out of the circuit (all 3 legs), you may be able to get the amp to power up to determine if there are any further problems.