2000 Dakota, how do I remove the battery tray?

You remove 3 bolts, only to discover a 4th one hidden behind some round thing that I can not identify. I figured out that the sensor at the bottom of the battery tray is a temperature sensor, but what the heck is that thing on the side near the firewall?

Do I have to remove that box (fusebox??) to remove the battery tray?

If it matters, it's a 2000 Dakota V6 2WD.