Are you getting lights on the dash, chime sound when key is in the ignition, radio, headlights? Turn the key and nothing? You have been driving without the ground secured since you purchased it, correct? It's not out of the realm of possibilities that the battery was cooked because of a faulty ground. Essentially, you had the body ground tab touching metal. It explains the M+S and intermittent starting issues.

Whoever sold you the truck didn't have the battery hooked up correctly. I would make sure you have a properly attached ground wire running from the shock tower/chassis to the ground bar where you hooked up the body ground. Pull the starter as mentioned and bench test it. Check the battery at home (multimeter) if possible or just take it in again and see if its loosing charge or cooked.

 

@Windycity_rover , yes, I'm getting full lights on the dash, shift lever is illuminated, dings, radio, etc. Everything by value of appearances, is functioning as expected. The new starter arrived today, but I will pull the battery and existing starter to get them load tested at an indy shop nearby since I don't think any of the auto parts stores really do that anymore. If the weather holds I should be able to pull everything this evening.

 

UPDATE

Got out there and pulled the starter, drove it to the parts store and tested it. Dead as a door nail. I went ahead and picked up a new AMG battery to be safe and already had the new starter arrive yesterday.

Got back home and put everything together.

Went to connect the battery and now, not only is the starter engaging, but it’s cranking the motor! Without a key! I’m literally standing under the hood with the key in my pocket and the car is trying to start! Thank goodness it was in park or the damn thing probably would have driven over me.

Does anyone have any idea why the starter would engage without the key in the ignition?

 

Can you post a photo of your wiring on the starter?

I just replaced my starter with one from Oriellys and it had several spade terminals (whereas the original bosch only had a single spade) for the "signal" wire and the large battery cable terminal was able to touch one of those spades causing the same auto-start behavior. I had to bend that extra spade out of the way but I've been meaning to go back in there and just cut it off.

 

You are definitely having some bad luck. Could only be two things. Either the solenoid is being energized some way, as mentioned above for example, or the solenoid contacts are stuck closed.

Went to start my cousins front end loader some time ago and had to rap on the solenoid with a wrench in order to make it stop cranking, the contacts had stuck closed. Can check the solenoid contacts with an ohm meter, should show infinity, will show zero if stuck closed.

 

We did it folks!! @gecko951 was spot on! I was so excited reading your comment last night that I got up early this morning to try it out. You'll see from the picture I've attached that when I had the battery lead connected to the starter, it was twisted and making contact with #3. I had the signal wire attached to #1. This morning when I removed the battery lead, I could see evidence of sparking against #3. So instead I turned the battery lead down instead of up, and bent #1 far out of the way. Then I connected the signal wire to #2

I turned the key to prime the fuel pump and she fired up immediately! First try!

I want to give a huge shoutout to @Extinct @gecko951 @Windycity_rover and EVERYONE ELSE that helped me on this. Thank you so much guys

 

Great to hear it!