[paolo]

Darkpuppet, you can snip the wires down in your adapter! Infact, you can snip the ones on your aftermarket HU harness shorter too if you want. As for the inline fuse, you can keep it there, it wouldnt hurt. The stock harness in your car will handle the current from the headunit, so it wouldnt be a problem.
The extra wires for the power antenna and power amp are there because some models of ford cars may have needed them at one point, but in your case, you dont need them. You only need the one remote turn on wire on the back of your HU, and you can just feed it to your trunk.

Hope this helps.