A3 Airbag Light

You need access to VAG-COM or a dealer to check the error that triggered the airbag light and reset the warning. You may have removed an item and turned the ignition on during the installation of the sub. Even though you have reconnected everything you will still get the airbag warning until it is cleared by VAG-COM.

A typical example of this is that if the glovebox is removed without first disabling the passenger airbag (via the switch in the glovebox) then you will get the airbag warning light on.

Where in the country are you? You may have someone local who can help with VAG-COM and reset the warning for you. Alternatively it will be a trip to the dealer who will undoubtedly charge £50 for a few seconds labour to reset it for you.

Done exactly the same thing in the past.

Yeah, this happens when the ignition is switched when the glovebox has been removed, disconnecting the airbag lead.

No problems are caused, just needs resetting. Dealer should do this for you free of charge.