To gain access to the thermostat you will need to remove the top and front engine covers. You will need to remove the serpentine (ribbed) belt. Take a digital photo or draw a picture of the belt routing before you remove it so you will remember how it goes back on. Mark the direction of rotation on the belt so it can be put back on going in the same direction. Remove the belt by either using a 17mm wrench or socket or a 10mm hex (depending on idler type) to loosen the tension on the idler (clockwise). Insert a nail to small drift in the alignment holes to hold the idler in the loosened position. Remove the ribbed belt. Remove the ribbed belt idler by loosening the 10mm hex head bolt that holds it to the engine block. This is done to facilitate removal of the outer left cam sprocket guard cover. You will then need to remove the lower left most front grill piece (on the front bumper) to gain access to the radiator drain. I use a long piece of 3/8
If this is a 2.8 V6 the thermostat is located on the front of the engine. Ask 'Where is the thermostat located on a 1995 Audi A6?' on this site and you'll find someone has done a great job of explaining how to find and replace the thermostat.
The thermostat on a 1998 2.4 Audi a6 is located on the radiator under the top hose assembly. To access it, you will have to partially drain the radiator.
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The battery is under the back seat
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You have to take off the front end of the engine, as if you were replacing the timing belt and water pump, huge effort for a thermostat.
The port is accesible from the backseat. Located under the ashtray in the center console.
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It is under the back seat.
Inside the hazard switch.
Rear window glass