Look at where the wire disappears. If it goes into a hole with other wires, it is probably bundled with them using tie wraps and you won't be able to use it as a messenger. I would do whatever it takes to expose and completely remove the old wire. I do a lot of marine wiring and my hot button is running across open ended wires that have been cut and let in place and are doing noting. It just makes for a sloppy job.
If you are going to go to the trouble to replace with new wire, use the best, marine grade, tinned, 6 gauge, quality crimp connectors and heat shrink on your connections. It only cost a little more to use quality materials, not the Home Depot stuff.
2015 Seaark 1872 MV CC Etec-90
Helix 12 CHIRP SI, HB 1199, MinnKota Riptide ST80/i-pilot Link, Bob's Hydraulic
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