I highly recommend the use of a proper dielectric grease on all connections on vehicles after the connection has been made. I install high current draw equipment on heavy vehicles, and always use dielectric grease. Connections covered with the grease have seen 2 or 3 years of service in a road salt enviornment and still look like new.
You could also try conductive grease, often a light grease filled with copper powder. It used to be available commonly as a anti-sieze compound for high heat applications like brake and exhaust fasteners, but has been replaced by synthetic compounds.
One other cheap thing that helps is to solder the lugs onto the wire, and cover the joint with adhesive lined heat-shink tubing. This keeps any corrosion from entering the wire-terminal joint. Then use "internal-external" lockwashers. These are the ones with small teeth on the inner and outer circumfrences, looking like a gear. Also, try to isolate the connection from vibration by tying off the cable close to the termination. This is important on vehicles, to help stop the vibrations loosening the connection fasteners.
As a last resort, I have had some luck cleaning oxidized wire with acid flux, used in plumbing. Dip the wire in the flux, then heat to the point where the acid flux does its job. Then clean the wire with plenty of water that has baking soda mixed in, to neutralize the acid. This often turns the copper a reddish colour, but atleast it is clean.