I've had the same intermittent fault with my LH indicator for a few weeks. After messing about trimming the dust cover and also swapping dust covers around, yesterday I finally decided to dismantle the actual switch with a view that if it was going to need replacing, nothing lost if I destroy it.
Anyway, all fixed and couldn't have been easier! Removed the rubber dust cap, then with a small screwdriver I popped off the button. I then removed the sprung plunger and there's two contacts ... these were clearly grubby and struggling to make a connection. I simply cleaned them up with some fine emery paper and also bent the contacts "in" a little to give a little more contact pressure. All push fit to reassemble and it works perfectly again. No cost, no soldering.
The switch isn't actually sealed so came apart really easily.
Stu.