How do run a second battery??

 

Just hook it up parallel with your first battery, + to + and - to -.

 

That or get a battery isolator so that you can run one battery for the stereo only and the other for the vehicle. The car charges the battery's as the current gets drawn.

 

isolators are bad as you will on average drop about 1 volt @ the second battery.
hooking 2 batts together is not good either unless they are the exact same batts because 1 will overcharge 1 will under charge
also be smart and do not put non gel batt inside your ride.