One dim parking light?

Sure seems like it.

Turn park lights on. Securely clip one lead of your meter to a GOOD battery ground point.

"Stab" the other probe directly into the socket shell of each park light. Set the meter down for low voltage measurements. I'd bet one or the other will read "above ground" by some voltage.

If that is not it, then it almost has to be one of two things:

Poor connection from the contact button on the bulb to the park power wire in the dim side

Or poor connection where the two park light wires splice into the single power feed in the harness......Not sure where that is, somewhere up where the harness comes "out of" the rad support area, I'd guess.