Not under the dash. Early VIN cars had a tap in the roof area where the microphone is located. However, my understanding is that the tap was omitted in current VIN production cars.
Most people will tell you that a radar detector does not work very well inside the passenger compartment of the Model S. The windshield has a coating that prevents the ardor signal from getting through. People that are serious about detecting radar have mounted the detector behind the from nosecone.