I want to move the battery in my brother's W116 300SD to the trunk. I have already figured out how to route the positive wire, but I need some suggestions of how exactly to do the other parts.

1. What gauge wire should I be using to go from the front of the car to the trunk? I am planning on to put an amp and subwoofer in the trunk to balance out the sound system (bigger alternator is on the way). Would 4awg be fine or should I go for 2awg?

2. What should I ground to? A hitch is bolted to the bottom of the spare tire hole (not technical term), would that be fine to bolt to?

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.