Most likely you have a bad ground, or corrosion on your power or ground connection. You're reading a difference in potential from point to point, you're seeing that your battery positive, batt negative, and chassis are at differrent potentials. I would be willing to bet you have some bad cable resistance caused by corrosion. Clean both battery terminals carefully before going any farther.
This sounds like the problem I had when my ground cable from my battery to my engine block became corroded at the block connection. This also sounds more like my GF's Frontier P/U when the ground terminal cracked and was loose.. Everything worked fine but it would barely crank (and woundn't catch or run) because of the current needed.

You need to use a multimeter, test lights are crap for actually figuring out anything useful. You are being confused by the limitations of what the test light is actually capable of testing/displaying.
When you use a multimeter, pay attention to whether you're reading positive or negative voltage between two points. I troubleshot my WRX in 10 seconds with a multimeter, I was reading 3 volts from my battery negative to my engine block, bad news. (there should be as little voltage difference as possible between those two points)