Where does the OE sub live on a B5 Avant...?

behind the boot trim on the left of the boot (opposite the fuel cap), if you take off the access panel, and look in the hole and towards the front og the car, the black plastic it the "BOX" that it is in.
 
IMAG0805_zps51fe5962.jpg
 
to be fair it's hardly a sub anyway, just a bit of mid-bass gets it sounding a bit nasty... [it's made of tupperware plastic]
you can see the side / top section just removing the pass. side cubbyhole cover without taling off the whole side panel.
 
Cheers peeps for responses, just checked and to my surprise its still there!

My next question, would a Bose sub from the S4/RS4 slot straight in?
 
No. With the standard setup the AMP if you can call it that feeds the rears speakers, and the head unit does the front speakers. With the Bose setup the AMP feeds all the speakers, so you would have to run the front speaker wires from the brown connector to the rear, and you would also need to run 2 wires for front left/right audio out to the AMP. Your also going to have to cut off the plug going into the current amp and and fit a new connect compatible with the Bose AMP.

You should also replace the speakers as the Bose speakers are 2ohm, and the standard speakers are 4ohm, they would work with the 4ohm speakers but it's best if you have the correct stup.
 
Bose is ****e anyway, buy a proper amplifier and proper subwoofer.

You can pick up decent quality kit on ebay for buttons.