Battery voltage drops to 5V with negative connected on a Ford Edge

The test lamp test posted above is a good start. I NEVER put a multimeter (in "current" or "amps" mode) in series with a battery until I have "judged" it with a test lamp, and if it lights bright, then use a stop lamp bulb. This gives you an idea how heavy the draw is. Many multimeters ARE NOT very well protected in the current mode, regardless of fuses.

Think about the stuff in your car that is "hot" with key removed. Alternator certainly, but nowadays, the PA transistors in the radio/ remote amp, dome circuit, cig lighter, and a few other things.