I had a the same issue so here's what I did.
- cut old wire off
- strip off outer cover(strip off just a little more than you'll need)
- flux both negative and positive strands thoroughly
- solder each wire, make sure the solder gets inside the strands
- measure each wires diameter making sure that each fits inside its respective hole, wire should have a snug fit
- measure and cut off excess wire
- if you have one insert outer protective cover over wire/cable
- loosen screw on plug, do not loosen to much or they will come off
- inside wires in their respective holes and tighten screw
When you tighten the screw it will dig itself into the solder and create a dimple as its tighhtened. The dimple will hold the wire in place and keep it from slipping out.
I went one step further and added marine goop to each hole where the wires fit and also gooped the section where the protective cover slips over the wire/cable. I insert and remove my plug every trip, its been almost a year since I did this and I have not had any issue to date.