Engine ground issues seem to be common. Fifteen years ago, my brother ran a wire from a relay in the power distribution center to the negative battery terminal in my Jeep. With this wire, the car started. Without it, it would not. He doesn't remember any of this, of course.

Now I get nothing when I turn the key to start. I've replaced said wire, I've replaced a few relays, no dice. When I turn the key to on and connect the two solenoid bolts, the starter just spins, no crank.

Am I likely correct to assume engine ground? Given these symptoms where's a good place to start?