My lady just bought a TT 2009, 2.0 TFSI. Similar issue as her old 1999, airbag warning light is on. The old one was a faulty seat connector that i fixed but this one is different.

Hooked up my VAG-COM and tried to clear the fault, but light came back instantly.
Can it be the key-switch that is faulty? The key seems to work (ON/OFF), the info says passenger air bag on or off on the dash.

This is the error code what VAG-COM throws up:

Address 15: Airbags

1 Fault Found:
01578 - Airbag Deactivation Warning Lamp; Passenger Side (K145)
009 - Open or Short to Ground - MIL ON
Freeze Frame:
Fault Status: 11101001
Fault Priority: 2
Fault Frequency: 1
Reset counter: 23
Mileage: 121331 km
Time Indication: 0
Date: 2015.02.25
Time: 19:41:13