I had a lot of problems with an airbag warning light after accidentally fusing it while wiring in a reverse light for a trailer with the ignition on (the manual even says not to do this, as an airbag failure will occur).
All it needed was an airbag reset (so said the manual), but the Fiat Dealer numpty, after being told what I had done still insisted it had a pass airbag fault, even though I have no pass airbag. So they messed around with the wiring, and hey presto afterwards I had an intermitent pass airbag fault.
It was eventually solved (and has stayed solved for the last 3 years) when one dealer checked the pass side load resistor and changed it. The garage that actually fixed it refused to charge me until the problem remained solved for at least a month!!! excellent service from Stoneacre Fiat again!!!!
Get it checked at a Fiat dealer, but be aware of people saying you need a new airbag ECU (as I was often told). Get them to remove the ECU, and clean the contacts and refit it. Then get them to check all the connections, clean them and refit. It is most likely a dirty connection or a load resistor problem.
Actually if the last owner was messing around with the hifi, it is highly likely that they removed the glovebox to access the cabling, and they could have either disturbed the passenger airbag wiring (if it has one), or the load resistor that is used in case it does not have a passenger airbag. The failure code from a Fiat Tester shoudl confirm the area to check.
good luck...