I believe the confusion stems from the wiring companies making harnesses that fit BOTH Camaros and Firebirds to cut down on expenses

Firebirds did not have front side marker lights in '68 due to the "wrap around" parking lights. Camaros had separate side markers and parking lights.

An example is 1st Gen Camaro tail lights. Some models have the backup lights in the tail lights. Others had the backup lights in the rear valance. You have to know what you have.

There are a lot of instances in parts catalogs for companies that sell both Camaro and Firebird parts that they list Camaro only parts for Firebird accidentally cause that's what the vendor told them.