want to run a 2nd battery in the back

[quins90lac 10+ year member]

I would put a second one at the back battery. the audio shop is treating it like a second amp. You need one in the front to protect the front battery and one at the back you protect the back battery. cause lets say you happen to cut the wire and it makes contact with ground well depending on where its at in the wire it could pop the front fuse or the back fuse even both. but since you have fuses the batteries wont be harmed and it might avoid a fire caused by an electrical shot.

 

[Villeza1 10+ year member]

I would put a second one at the back battery. the audio shop is treating it like a second amp. You need one in the front to protect the front battery and one at the back you protect the back battery. cause lets say you happen to cut the wire and it makes contact with ground well depending on where its at in the wire it could pop the front fuse or the back fuse even both. but since you have fuses the batteries wont be harmed and it might avoid a fire caused by an electrical shot.

True, better safe than sorry I always say.

 

[audiolife 5,000+ posts]

If you have a battery in back and 1 up front you need a fuse within 18" of each battery. If you just fuse the front battery and the short happens after that fuse you still have a battery that will arc and could start a fire. Being that you have a full frame vehicle (at least what it is in your pic) you may get away with grounding to the frame in back. In most cases in a unibody car I wouldnt waste my time with trying a frame ground. The CRX we just built took about 5 hours and the majority of time was running 6 runs of 0 ga front to back and upgrading the front battery ground with a copper bar. Time and money spent doing it right is much cheaper than replacing an amp or frying your electrical.