Temperature has to be below 5 degrees and the HVAC needs to be on 'Auto'. If you have access to VCDS then you can scan it for errors and also use it to perform the Output Tests on the HVAC system. If the conditions are right then the amber light should flash on the button when in use.
As for fuses, I don't believe there are any (or at least none that I can find) as the control unit fitted in the boot is wired directly to the battery. The battery also needs to be in relatively good condition as it draws a lot of power.
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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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