If it's like mine, you can't replace the bulbs. The lights are LEDs on strips glued into the covers. If one side is working and one not, you probably have a loose or broken wire on that side. The ones I have are soldered in to the end of each strip. Remove the bolts holding your covers on and look inside the edge where the wire(s) go. Check the solder joint and the wire where it's attached. I,ve had to re-solder one side of mine and re-route the wires.