I bought a 1990 Chevy 3500 a few months ago, and I've finally got around to trying to figure out what the other battery in the engine compartment is for. It's obviously designed to be in there, but it's not connected to anything. It's a blue-topped Optima. The negative side is connected to ground, but there is no wires connected to the positive side.

In the engine compartment near this battery are 3 wires bundled together and tied up out of the way. One is a short black wire; both ends are in the engine compartment and neither end is connected. The next wire is blue and the end in the engine compartment is not connected (looks like it was cut some time ago). The last wire is red and the end in the engine compartment is not connected (it also has an in-line fuse on the red wire).

The red and blue wires both run together to just foward of the rear bumper. On the end of the wires is a label that states: "RED - PWR FEED" and "BLU-DK-AUX".

Was this suppose to be some type of auxiliary battery for trailers? Obviously someone spent some money for this battery, but I have no clue what to do with it.