You cannot really re-condition wire . The oils in the insulation deteriorate over time and that is what makes the jacket brittle and crack. Armour-all may help if it is not dried out too bad.
New stranded commercial electrical wiring is insulation class THHW / THHN-2 which is gasoline and oil resistant and somewhat sunlight resistant.
I added a ground wire to the frame of the back-up light and on each one of the turn signals and grounded each to the frame.
A good heavy wire with soldered connections(and heat shrink tube) clean sockets and a good ground will have reliable bright lights . Most lighting problems are usually due to bad grounds and poor connections on old trucks.
