My mig wire melts inside base unit when trigger is pulled

The above information is not correct.

The entire wire is polarized by the tip during normal operation, making the entire supply of wire is polarized to whichever polarity is selected. The cabinet of the welder is usually bonded to the output, and so therefore is the spindle on which the reel is kept.

You are using a spool that has the wire anchored on the outside of the reel, rubbing on the spindle assembly, and making a sliding contact.

Put an insulator between them. A circle of thick cardboard (not corrugated) with a hole in the center can work well, as do plastic lids- Cottage cheese, tub of grease...

Your liner is grounding some place make shure the tip is not touching any thing but the answer to your question is there is a direct short in your feed hose or tip or liner