One dim parking light?

You should be able to take two new bulbs and do the following:

Put a working old bulb in either side

Take your two new bulbs and try them in the opposite side. Both should be "apparently" the same brightness.

Switch your old bulb to the opposite side. Again try both new bulbs in the remaining side.

Now install both new bulbs one in each side. Still a problem? If so It HAS to be

socket grounding

wire pigtail contact

splice in the harness.

I assume you are comparing brightness with the lenses OFF. One lens darker? From sun, fading, etc?