Car has no power. It started cranking then acted like it popped a fuse. I have 13 volts at battery then I have 5 volts at the junction box. Wtf. No fuses are blown
Does the battery relocate have a ground wire running all the way to the engine bay or only in the trunk? If there are no spices in the positive wire running forward then I would think it is a ground problem. The load it takes to crank over an engine can show all the weak spots in your wiring. Chased a problem like this on my work truck for nearly 6 months. Eventually found it cause the problem spot started smoking while trying to crank the engine. This was with 3 batteries on a diesel, but could be similar all the same
edit... Quick way to check to see if it is a bad ground would be to use the negative side only of a set of jumper cables and hook one end to the negative side of the battery and the other to somewhere for a ground on the engine block. That should let you know if it is a ground problem or not