Tin coating is used as a corrosion inhibitor, Even copper when subject to weather and moisture will form oxides and most likely sulfates, blue/green colors, which adds resistance to terminations. Automotive wiring subject to salts and corrosives, would do well to be tin plated, IMO. There are also additives, pastes, that can be use to increase/protect terminations in addition to the plating. Some crimp connectors will have the paste include as part of the connector. A 500 ft roll of single conductor, THHN, 12, 14, 16, or 18 ga copper, either stranded or solid, should be available at a local electrical supply stores. There are also color choices. Surprisingly 500 ft does go far. This forum also has wire for sale. Note that stranded has a slightly larger diameter.