i dont think there is a adapter for a stock radio, if you have the tuner already you can buy the antenna adapter that plugs inline on the antenna cable, that way you still tune it to a station but you wont have a problem with other stations interfering because the signal is so strong.
also you could buy a aftermarket stereo with a aux input or a sirius add on tuner for direct sound
i supposed you also wire a amp and switch the inputs between your stock deck and the sirius as well.
from what i understand the tuner cable for 20.00 works very well and thats probably what ill do for mine, i still use the transmitter built in and it sucks when i travel with it (having to change stations constantly)
also some people on ebay sell stronger transmitters that are supposed to be much stronger than the transmitter built into the sirius recievers...