Bad starter? Short? Switch?

I recently replaced the starter in my 95 Chevy c1500 pu. Last nite after leaving the gas station, while sitting at a red light I noticed a whirring sound. I shut the key off, and the whirring continued. I realized it was the starter still turning. Just then the light changed and I tried to start the truck. The dash lights dimmed but no click, no nothin. Luckily its a manual trans, so a quick push got us out of the intersection. As soon as we were clear of traffic, I disconnected the battery cable and it finally stopped. I was able to get it home and promptly dropped the starter and traced down all the wires. Every thing checked out fine, except for the wire from the dist to the alt. It was laying on the valve cover and was showing some copper. I replaced the damaged section of wire, put the starter back in, and tried to reconnect the battery. Big spark, starter turning, but not engaging. I shouldve bench tested the starter, but it was late and since it was brand new and not a rebuild only in the truck a few weeks. I can still start the truck popping the clutch, hot cable to battery disconnected. If I turn the key when its running, the starter engages and grinds against the flywheel. I suspected the ign switch itself, but seems more like a short. I would think with the hot cable discon, it wouldnt try to engage. Im stuck.