I have been using cheap thinner and a rag to wipe down all my wiring harnesses in the restorations over the years. Yes some of the paint has been on the wires for over 20 years.I clean them a few times until the wires look like new. It removes old oil and grease along with the paints. I use an old tooth brush to scrub the end connectors with the thinner. After cleaning I always check each wire with an ohm meter to see if there are any breaks in the wiring harness. The wires get brittle with age. This method has worked for years of cleaning. Hope this helps.