I have an Optima Yellow top battery installed under my GS hood and would like to add another one in the back of the trunk.

How and what option do I have to run its wiring ?
To go parallel wiring ? Do I need a capacitor between the positive
pole ?

 

You can do it two ways, one more accepted than the other. The most accepted way is to go purchase a battery isolator, from either a Marine shop, or possibly a auto parts store. Your second option is to just run them parrallel, which I have seen done without issue. Personally, I would go buy the isolator because it will protect your charging system from damage. Make sure you use matching batteries.