The blue wire with the yellow end is the fusible link that protects the entire system from overload and fire. Normally, cars use a 10-gauge power lead and a fusible link that is four sizes smaller (14 gauge). This is a special wire that has insulation that allows oxygen in so the wire will burn when it gets hot. If you experience a bad fusible link, or you know it's bad, under no circumstances should you replace this link with a common wire. Use only another properly sized, fusible link.