73 Dart Side markers no longer work
slowly converting to LED lights
Be a lot (a
lot) more careful with this than it's easy/common to be; the overwhelming majority of "LED bulbs" and "LED headlamps" on the market are fraudulent, unsafe junk. There are a few worthy products, too, but just going by website hype you are just about guaranteed to get bad picks.
For the side markers I went with the black ones that have a colored LED strip that lights up when they turn on
You might or might not care, but the requirement on vehicles made after Dec 31 '69 is for side marker lights
and reflectorsl, red rear and amber front. Not "or" but
and. The reflectors do a great job of keeping you from getting sideswiped or T-boned while parked, and they can also be lifesavers/carsavers in specific situations that aren't very common but are very bad.
Also keep in mind that while it's possible you have a wiring/ground problem, the marker-things you bought are not of anywhere near OE quality, so it's also possible you have faulty lights.