Right now im running 2 batteries. One for my starting and the other for my system and a perko switch. Im going to add a 3rd battery for my stereo system. How do I hook it up? Do I just run battery cable to positive and Neg to my existing stereo battery? Or do I have to ground it also to the block? Also do I to change any wiring to allow for the extra battery?

The simpliest way is to wire your two stereo batteries together in parallel; that is, positive to positive and negative to negative with your starting battery grounded to the engine block (common ground). <BR> <BR>As far as wiring, I wouldn't use anything less than 1/0 for all of the connections.

right now I have 4 guage. So you think I need to upgrade?

I don't have the calculations for you. However, for peace of mind, I would replace with 0 guage power wire. You can get Stinger HPM 0 guage power wire from darvex.com for $2.95 per foot. This is the best price I've seen by far.

Do you sit for long periods with your radio on? If not look into an alt upgrade. there is a calculator that will show you based on Ohm's law and wire length what guage wire you would need. how many amps is your system pulling?