dull driver's headlamp & driver's side markers not working

Good Saturday afternoon! I'm re-assembling the 1970 GT after the re-spray and bought some Lucas PL headlamps with new pigtails. Connecting up the lights and re-connecting the sidemarkers has resulted in lots of frustration! The driver's side halogen is dull and the side markers don't illuminate. Is this likely a ground? the passenger side low and high beam work well, the driver's side is about half as bright.
Any idea where to start troubleshooting? Thanks in advance!

Sounds like the ground connection, or the black ground wire for that side.

Also a good time to put in relays for brighter lights. See the posts just above yours.

Must need some polishing and cleaning of your electrical contacts and/or a relay for the headlamps. Sometimes a little scraping with a knife blade or some sanding of each contact/plug will help. There's also electric cleaner spray you might try on the contacts, as well. Oh, before you begin, disconnect the battery. Exclamation points here.

And check that all grounds are tight. Sometimes they "look" fine but when you unscrew them, you discover they really weren't.

Work methodically on each one, including the grounding wire. Unless the headlamp itself is fogged over, this is most likely the problem. These cars "leak electricity"! And oil. :) Or you might have blown a fuse. Check those, too.

Update! The fault was a wire (blue and white) from the headlight to the wiring harness that had rubbed the insulation off and was grounding out on the hole in the inner fender. I have sanded and dilectric greased all the bullet connectors and patched the insulation, and we're back in business! Thanks to all who responded and I will look into the relays!