Relay issues, Adding a fan for amplifier

I am adding a amp fan for my amplifier in my boat.  I have bought an bosch realy and socket and am going to wire the remote turn on lead from my radio to the relay which will then turn on my amp and the fan.  I have the diagram from this website describing the necessary wiring however I have a few questions.  On the 12V+ lead that is connected to terminal 87, do I have to run a separted wire to my positive battery terminal or can I use the 12+ wire that is already ran to my amp(Basically the amp and the relay would share the 12V + lead)?  On the ground wire that is connected to terminal 85, do I have to run a seperate ground wire to the negative battery terminal or engine block or can I just use the ground wire that is already connected to my amp.  If I can use these wires on my amp can I just connect them directly to the amps terminal where the 12V+ and ground wires are already connected or do I need to splice the 12V+ and ground so that on end of each wire goes to amp and relay?  As you can see I am basically trying to avoid having to run extra wires to the back of my boat.  Any imput much appreciated.

any steady 12v source will do and any ground will do.As long as the wire is of ample size to handle the load and As long as the fan is brushless.

A normal fan like your heater uses will cause noise in the audio section of your Amp.