We noticed last night that the running light on our friends 99 softail standard was out. The brake light still works along with his signals. Everything yo front works as well. I'm guessing it's the bulb seeing as it's a dual element but just wanted to get some input i case I'm on the wrong path.
Thanks in advance!

 

Bulb is most likely source of the problem especially if the front marker lights are still operational.

 

Dual filament bulb with the running light filament burned out. Quick and easy fix.

 

This. The bulb has 2 filaments. Running light goes out first cause it runs more. Put in new bulbs (replacements from any auto parts store) and you'll be all set.

 

I replaced mine with a Radiance LED setup on my Softail deluxe because the bulb is too dim on the tombstone tail light. The LEDs were in a disk with the wires going to a standard dual filament socket.