Check posts #9 and #10 in the following thread. The thread is about the D3 but post #9 references the D2 and includes photos. Use a multimeter to check resistance on the two sensors. Post #10 mentions a contact cleaner for the airbag connector block:
https://www.audiworld.com/forums/a8-.../#post24456884
"N201 Rear side airbag igniter, driver's side" - This is what the current flow diagram gives for N201, but I think "driver's side" in this instance refers to LHD vehicles. MJ will confirm, but odd part numbers are 'left' side and even part numbers are 'right' side, irrespective of LHD/RHD.
Just found the following in the current flow diagram, which would rule out curtain airbags:
N251 Curtain airbag igniter, driver's side
N252 Curtain airbag igniter, front passenger's side
I got these from the following PDF in the D2 Tech Dump thread:
Current Flow Basic equipment From model year 2001.pdf
__________________
2001 A8 D2 FL 3.7 Q - LPG Conversion, Engine: AKC, Gearbox:FBB, Colour:LY5X Aqua Blue pearl effect, Interior:WJ light beige/dark beige valcona leather with burr walnut inserts
My DIY jobs -
Replaced alarm horn,
Replaced Coolant Temp sensor,
Updated RNS-D firmware,
Installed Reversing Camera and Digital TV,
Dashcam Installation,
Retrofitted Rear Blinds,
Auto-Dimming Rear View Mirror,
Chrome Boot Struts
Sold! 
2008 S8 D3 5.2 V10 - Engine: BSM, Gearbox:KLW, Colour:LY7F Suzuka Grey Pearl Effect, Interior:Black/Black with Silver/Black valcona leather with carbon atlas inserts, Bang & Olufsen, Heated windscreen, Soft-close doors, TV, DAB, AMI, Quad Zone, Auto Boot, Auto dim Mirrors, Keyless entry, PDC with rearview camera, AFS II Headlights
My DIY jobs -
Fan Jet Windscreen Washers,
Retrofitted Electric Side Blinds