My left brake tail light melted - any idea what could cause this?

Check to be sure the bulb is the correct number. If it's too large (physically) it may be too close to the lens, or if it is too high a wattage for the light, it would just get too hot, and could melt the plastic lens. Compare the bulb to the specification in your owner's manual, and to the bulb in the other taillight.

I have the same problem, original bulb melted taillight.