...and it's not the bulb. Any suggestions? ('09 Eclipse Spyder)

 

What it could be is the wires powering it. Check for power and ground. I'm sure there are plenty of diagrams on the internet to help you out with that. Also, it could be the connector, might have a buildup of corrosion (seeing as it's an 09 thats hard to believe) at the pins, or the wires might have high resistance, which can be as bad as having an open. I don't think it could be the fuse because the one fuse that's for the headlights, goes to both of them, so they'd both be out if it was the fuse. So check for those three things dude: power and ground, corrosion, and resistance. If you have a DMM/DVOM, this helps a lot! Hope this helped dude.