Ugh, that's a tough one.
A stereo installation shouldn't have affected your airbag mechanism or wiring. However, may have cycled the key while the passenger airbag indicator light was disconnected. This would set a error code in the SRS system.
Two options: get a scan tool and erase it yourself, or have the dealership do it (assuming it's a history code, which is likely). The dealership shouldn't charge for it - I'm guessing you got a "slow" tech when you went in before.
You can also clear a history code by disconnecting both battery cables, and touching them together for a full minute. Hook them back up, connecting positive FIRST.
If it's a current code you may have damaged something by accident during install, but I'll bet it's just a history code.