So before I go tracing wires... is it "normal" for one of the brake bulbs to be more dim than the other? I noticed it this weekend when I was swapping out bulbs. Thanks
Disassemble, inspect, clean, reassemble, verify voltage (when the bulb is operating, not when no load). The "solder balls" on the bulbs can also get deformed/oxidized/corroded.
Check to see that you have the same wattage bulb in both sides. I remember this because a friend living down the street had different light levels from the brake lights on his Sunbeam Alpine, both bulbs were the same # but different wattage. A lesson learned and remembered from 50 years ago.
So it looks like it's the holder. Directly wired (with alligator clips) the "dim" bulb while the other was in the holder and it works just fine. My next step is to re-wire the holder since they don't appear to sell one separately. I did find that it appears that they didn't solder the ends of the connectors. Looks like a non-insulated spade connector that was cut down and bent over for the connections. So I'll go that route. I just finished installing my new radio after that I was done with wiring for the day.