Inner panel below cowl sides near door hinge area. Lower side attaches to rocker panel. Often rusted.
This fits:
1966 – 1970 Dodge Charger
1966 – 1970 Dodge Coronet
1968 – 1970 Dodge Super Bee
1966 – 1970 Plymouth Belvedere
1967 – 1970 Plymouth GTX
1968 – 1970 Plymouth Road Runner
1966 – 1970 Plymouth Satellite
1970 Plymouth Superbird