LilRoush

Yeah those lenses are very legal and look like they are in great shape for the age.

I'd look at bulb output (voltage or correct bulb being used). A loose ground can dim a light like you are seeing. Check the back of the socket to make sure the wires are connected tightly.

Old Mustanger

The guys are right,
If you have no tint,
And you have already cleaned/polished the lenses,
Then the only thing left is light bulb output.

You most likely have a wiring issue and the bulbs are not getting full voltage. This can be a bad ground wire/connection or hot wire connection.

Did your year Mustang have a relay to supply full voltage to the rear lights? If so, the relay may be bad.
If you don't have a light relay, the switch may be bad. This would be in top of the steering column. It gives you one more thing to test/eliminate. I have seen these go bad before when there wasn't a relay used, because the switch has to run the full amperage of all the lights.

Look up a wiring diagram for your Mustang, this will quickly show if you have relays in your lighting circuits.