The passenger headlight is dim when on, bought new bulb, it did not fix it?

I was working on a 2008 Chevrolet Cobalt. Passanger light was really dim. I checked all fuses and they were all fine. I followed the wiring and leads to the fuse box. Just as I was getting to the fuse box. There is a single ground wire that comes out of harness and screwed on driver side headlight frame. Wire was loose as soon as I touched it my passanger light went bright again. I tighten it and on the road it went!

Have the voltage and ground tested at the headlight plug, could be the plug (or connector) is the culprit.