Mustang American Autowire Wiring Harness (1965-1966 Mustang American Autowire Wiring Harness (1965-1966) Installation Instructions. 1965-1966 Mustang American Autowire Complete Wiring Harness Classic Update Kit The routing of the engine bay and headlight wires can run up against the firewall and through the grommets we installed earlier. The driver’s grommet is for lights and middle 66 mustang engine wiring harness | eBay Save 66 mustang engine wiring harness to get e-mail alerts and updates on your eBay Feed. + Items in search results. SPONSORED. Find the right parts for your 1966 Mustang. 65 66 Mustang Engine Bay Headlight Wiring Harness Original 1965 1966. Pre-Owned. $50. Buy It Now +$29 shipping.