Mustang firewalls from ’65-’73 generally look like this, with one opening for the factory fan motor and two passages for heater hoses. Between the heater hose passages and fan motor opening is the evaporator drain passage located 13⁄8 inches toward the tunnel and 5⁄8 inches down from the heater attachment bolt hole. There’s a template in the kit to help you get it right. Classic Auto Air recommends a 5⁄8-inch hole for the drain hose. Use a rubber grommet around it to prevent chaffing.