You have an intermittent short. Meter all your fuses for continuity. The lightbar fuse is a ten amper: #1. Take off the lens for each light. Start with the left light. Inspect the wiring for a bare section where it is contacting the case. If you can't find anything, Then: See if you can isolate a section of wire where the direct shorting occurs. Might try something like this: take the left bulb out. With the right bulb in does the fuse blow? No, Replace the left bulb and take out the right bulb. Blow the fuse? This will help identify which side of the lightbar wiring has the nicked wire. Also the harness itself ( between the light bar and the relay) may be shorting out.
I'm writing this because i had the same problem you have. Turns out the wire in the left light bucket was shorting out on the case. Annoying too because the nature of the short was intermittent due to the wire end vibrating ever so slightly and hitting some metal in the light bar's left light bucket. In the left bucket you have the switch as well. Check those leads too.
