Just saw this thread for the first time ...
As reported in
this thread last May, started by an incredibly handsome, articulate, intelligent, etc., etc., etc., Flyertalker, Avis started having the ability to activate XM Radio on a per rental basis at that time ... it appears that National (and I would assume by extension Enterprise and Alamo) are implementing a similar capability.
As I said in that thread, I don't fault both the car rental companies and XM/Sirius for doing this, but my problem with it is quite frankly the charges are ridiculous ...
this should be $1 per day ... maybe $2 per day ... but the prices they are seeking are crazy.
I really wish, as some discuss in this thread, it was easy to just activate a rental car with an existing subscription ... I only have five active subscriptions (three lifetime XM, one lifetime Sirius, and a one-year trial on my new BMW), but I have to use a portable unit or stream in rental cars, even if they have their own receiver.