I have the same problem with my car, in that i removed the car stereo connected up the back speakers and then I noticed the airbag light on. the dealer reset it under warranty but it was back on within 24hrs. They tell me that there must be a cut wire somewhere but dont when that might have happened. They've also informed me that the ECU stores info regarding crashes, which my car has been in. I reckon he's using it as an excuse to say he doesnt know what the problem is. They want me to book the car in for a day to find the fault.
Yeh like i'm made of money.
I'm getting the vagcom stuff tomorrow, because i need a key reprogramming, my dashboard lights flicker occassionally and the temp sensor drops to zero and back up again intermittently, so i'll see what that shows regarding the airbag light and stuff and the details regarding the crash. I'll hopefully be able to figure out how to remove your light for a Xmas drink.
I'm based in Walsall, West Mids.