Like you already found out, get a good ground to that amp. 0 gauge is way too thick of a cable and hard to work with. I would get a 4 awg kit instead. Make sure it says 4 AWG and not 4 gauge. It will be anywhere $40-$75. Connect straight to battery with the kit's fuse 1 foot or less from the battery positive post. Positive to red post and Negative to black post of the battery. With the extra 4 awg cable, wire the engine with the body of the car. The strut/shock tower will be the best spot. Remove one nut that holds the strut in place and scrape the paint off until you reach clean metal. Crimp a terminal onto the cable tip and place into the tower bolt, washer on top of that, then the nut on top of that, tighten. Now do the same with a bolt on the engine. It may be hard to find one but is important. This will make sure that 100% of the power reaches your battery. This pic is the best I could find. Red is the wire and tips are bolts.
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