M1008 starting problem

Here is my issue, any help much appreciated...truck would not start, front battery low and charging and now to 12.4, now when negative is put back on front battery, the starter is activated and this happens without the key (without the key put on “on” position), also a fuse-able link in a wire going to left alternator has blown and the wire is still hot, I’m not sure what to do next because I can’t complete battery wire assembly because of the bad sprark that it causes and subsequent starter activation.....I did preview some threads that seemed Similar problem and the advice was to replace starter solenoid and it may be stuck and defective, so is this the starter solenoid on the starter or a solenoid/relay under the dash?...thank you

Most likely it's the one under the dash. It will be bolted to a plate that has 2 small relays attached as well . Just pull the plug out and see if that stops the cranking.

A lot of people on the site change out the stock relay for a heavier duty one as shown here:
https://www.steelsoldiers.com/showthread.php?60960-doghead-starter-relay-modifcation

A low voltage situation will cause a lot of harm to your starting system. Most likely the starter solenoid welded itself together 'regardless" if its heavy duty or not. Always keep good battery charge or it will rear its ugly head and cause damage to the solenoids, relays, glow plug system, and evan the starter.