call me crazy but i cannot find any where at sirius.com
info on 2nd and third radios on one account at 6.99
this is from terms and conditions at sirius


2. Multiple Receivers: Each Subscription to the Satellite Radio Service is tied to one Receiver. If you want to have the Satellite Radio Service on multiple Receivers, you must purchase a separate Subscription for each Receiver although all of your Subscriptions may be combined on a single account.