Greetings, Nice looking car by the way.

My headlights on the 36 chevy were a bit confusing to get to the bulbs too. So maybe it is the same way on your plymouth.

There is a small hole underneath the front ring. If you take a small slotted screw driver, you will notice a slotted screw that you can only get to by having the screw driver unscrew it from the side of the light fixture.

Once the front ring is lose enough, it will unhook from a top clip and slide right off.

Then you will see (likely) three small slotted screws that will hold the reflector in place. Once those screws are off, the entire reflector assembly is free.

You don't need to unbolt the whole assembly from the fendor to do this, I've done it several times once I figured it out.

Hope this helps, let us know.
Jerome