What causes a fuse to blow is too much amperage draw. This could be cause by a short (voltage straight to ground without anything providing a voltage drop), high resistance (bad connection causing the system to work harder and demanding more), or the system itself could have a component in it that is bad (shorted). When does the fuse blow? If it blows as soon as you turn the key then probably you have a short. Could be the insulation chaffed and now you have bare wire touching ground (anything metal that is bolted to the chassis), or the system itself is bad. Probably wiring though.
If it blows after you have been listening to the system for a few minutes or so then you may have high resistance (bad connection), or again a faulty unit.
I bet you have a wiring prob, bad splice where you crimped to hard broke the plastic and now the metal of the splice is touching the metal of the car (short), or a splice that is not making a good connection. Maybe only 4-6 strands of the wire are actually crimped and not the whole wire (high resistance).
Like the other poster mentioned you do not want to grotto a higher rated fuse. A fuse is a safety device so Pioneer says this unit is safe and built to work off of for 2 amps (for whatever that wire is feeding inside the HU) and if you go and put a 10 amp fuse in it you could end up frying your HU or causing a fire.
Hope you get it fixed.