OK, So in most (if not all) cases when you install S3 seats in an A3, you can't simply plug the airbags on the seats into the wiring on the car.
Often the car has a three-wire firing mechanism and the airbags on the S3 seats are two wire. From what I can work out looking at the wiring diagrams there is perhaps a fundamental difference in the way the bags are fired - so if you choose to attempt to modifiy either end of the loom and connect the seats that's up to you. If both the bags in the seats fire and you're left covered in expensive leather/alcantara and foam, don't say I didn't warn you!
So, without the airbags connected, the A3 will throw a fault code and the light on the dash is illuminated. This causes two things; firstly, it's damn annoying. Secondly, and perhaps more importantly, from what I can work out it means that the airbag controller won't even attempt to fire any of the other airbags in the car.
This means that in an ideal world it would be possible to somehow 'confuse' the car into thinking that the seat airbags are connected, clear the fault code, and everything should be hunky-dory. No warning light and working dashboard airbags.
OK, so ste_1210 got in touch with me as I'd done some investigating into this issue a while ago; and the other night we were chatting away with him in front of his car and me in front of a pile of wiring diagrams for the A3 and S3.
We eventually decided that the 'resistor method' would be the most likely fix; and certainly the best thing to try first. For those who haven't seen the other threads, this involves bridging some of the wiring loom on the car with a resistor of a suitable value so that it thinks the airbags are present.
After a bit of checking diagrams and discussion between us, we worked out the following...
Each side (driver and passenger) has three wires in the plug for the seat.
...The Brown wire on each side does literally just go straight to earth, the remaining two feed back to the airbag controller.
Based on that information we figured that the plan is to ignore the earth connection and bridge the other two on each side with a resistor.
Based on previous discussion, 4.7Ω looked like the most likely value for the resistor; so ste_1210 got himself down to Maplin and picked up some resistors. He bridged the two pins in the plug with a 2W 4.7Ω resistor, and used VAG-COM to try to clear the fault code.
...The code cleared, the light went out!
So, while some of the details may change depending upon the MY of your car, and indeed the seats (wiring colours etc.), the theory is sound and it is possible to get around this.
I guess I have to make a disclaimer and say that if you want to try this procedure, it is of course at your own risk - especially if there are any variations on what we found; but we can say with confidence that it works - and I'm more than happy to try to help others out with any variations they may find.