All automotive wiring outside of the passenger compartment should be environmentally sealed. Heating and cooling cycles, as well as capillary action, can draw moisture, salt, and road grime deep into the wire. Someone had repaired this section of wire with an insulated butt splice terminal. It was also wrapped with electrical tape. I stripped about 8 inches of insulation from the wire and found it black and corroded.
I like to seal my wiring repairs with adhesive-lined heat shrink tubing.
I spliced this wire with an un-insulated butt splice terminal. I used a heat gun to seal the repair with heat shrink tubing. Note how the adhesive lining has melted and oozed out of the end of the tubing.