The small terminal on the starter solenoid is very close to the main terminal that the battery cable hooks to. If that cable end lug touches the small terminal, or the wire to the small terminal is touching the main terminal, it'll trigger the starter to crank whenever the battery is hooked up. This happened to me on my R2500 square body Burb, scared the hell outta me. These two terminals are sometimes separated by a ridge on the plastic cap of the solenoid; not all solenoids are made this way. Compounding the problem is that there's not much room to maneuver unless the truck is on a lift. But I agree with the other posters that you definitely need to look at the cables. They can be corroded badly and it's not visible, and the factory loop hold downs are there to keep the cable away from the exhaust manifold and the suspension. They're a bother to put the cable through but worth the effort to keep your cable safe. On my 99 Burb there's 2 positive cables and the factory replacement ones are over $200. I had a battery shop make a replacement one for me for a lot cheaper. Good luck, let us know what finally fixed it