Can I connect a radio to the battery?

true....i could understand re running the constant (some reason couldnt get power to come through)...had to do that one time in my old honda. but not sure why you would want to do that on power.

Without getting too technical the purpose of using a fuse in this case is to protect the wire...in other words even though the deck may have it's own fuse it is designed to be connected to the factory wiring at the dash..which is fused as well at the factory fuse panal, and the main feed from the battery to the ignition harness is "main" fused as well.

If you run a wire from the deck to the battery this presents a hazard if the WIRE itself is not fused close to the battery. If any part of the wire between the deck and battery were to short out in any way (pinched, screwed into, isulation peels off etc.) this will cause the wire to burn as the FULL current available from the battery travels through it. This burning will continue until the wire either breaks, or burns to the point where it's atoms no longer resemble a conductor..so yes...danger.

A simple 25 cent fuse at the battery will prevent this. The one that is already used with the deck will not protect against this, that is there to protect the deck itself from burning up..the one you need to put by the battery protects the wire and in effect the vehicle itself.

Hope that cleared things up a bit.