When you say it's looking worse for wear, is it the silver knob itself or the black centre part on top. If the latter, I'm seen several posts saying that you can just peel it off and there's another layer below it (can't verify that though). If it's the former, then check out the following links...
Photos of underside of MMI unit with instructions on how to remove. Link is to that actual post within the overall thread:
https://www.audiworld.com/forums/a6-.../#post24413071
A post on how to remove the knob. Again, linking to actual post within thread:
https://www.audiworld.com/forums/a6-.../#post22552383
Finally, part number is 4F0919069A. Dealer cost around £35, and anywhere from £9 up on eBay.
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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,
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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