67 barracuda tail lite question.

4 factors that make those lights dim,
Lenses darkened with age,
Little or no reflective properly in the cavity
A single 1157 bilb in such a large fixture
Current loss ,less than 12 volts reaching the bulb and weak chassis ground path.
There is a Sylvania long life bulb that is slightly brighter, has a nickle plated base and a different number that I forget.
Many will suggest a LED
My solution was to butcher the fixture enough to add bulb sockets similar to the 68 fixture design.