I just picked up a Sirius radio and will be installing it this weekend. What I'm going to do is use an add-a-circuit and run a wire from the fuse box to a 12v accessory outlet that I'll hide behind the dash and plug my sirius into. I've also read that others have used the add-a-circuit and cut the accessory plug off the end of the radio power adapter and connected it directly. I probably won't do that as my radio says that hard-wiring it directly without the accessory adapter will void the warranty and cause damage to the unit. There are a couple threads here outlining the steps for both scenarios, I'll try to dig them up and post them......
