It's not the bulbs since they either both work or they both don't.
Whenever it's raining, even a light drizzle, they don't work. Only on the left side.
This happens not even a couple miles down the road.
When the weather gets dry again they're back to normal.
This happen to anyone else?

2001 FI Fat Boy

 

 

If your bike is parked in the rain on those days when the left lights don't work, I'd suspect the wiring or switch in the left handlebar switch assembly is the problem. If your bike is garaged in the dry, but the lights stop working once you're riding, that is a bit more obscure! In that situation, before setting off into the rain, try your lights to see if they are OK, then again once underway. In addition to Ed's suggestion, I suggest you dismantle the offending light, plus handlebar switch, and check the grounds and wiring connections. Make sure everything is clean, tight and undamaged. Use dialectric grease to reassemble the grounds and connectors.

Many of these frustrating electrical problems are actually mechanical - in other words things like a damaged wire, dirty or rusty connector/ground, which are often easily found and put right - without needing a degree in computer electronics!