I'm not 100% positive on the 2nd gens, but I believe the airbag recall was only for the passenger side and for the airbag computer thing. They shouldn't have touched anything on the driver side, but I could be wrong.
With that said, the majority of the time on the 1st gen (03-08) if the horn stops working it is simply the clock spring, or spiral cable I think Toyota/GM call it. They can be purchased relatively cheap from Monkey Wrench Racing or eBay. I'd like to think it would probably be similar on the 2nd gen.
Is your SRS light on? If so, have you pulled the codes?