They are indeed a very common failure issue on the D3. I replaced both of mine when I had the S8 with secondhand ones, and decided to dismantle the old one. Basically all it consists of is a large heatsink with four transistors bolted to it, with the heat from the transistors being dissipated by the heat sink, and with some air blown through them you get a very basic heater. It's all held in with some sort of epoxy, so you'd end up destroying the outer part trying to get the heat sink out.
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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, 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
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