Thanks to mogur and mwalsh for telling me about the traffic information. That sounds useful enough that I at least wanted to give it a real trial. So I called XM CS (phone number on the radio). After a long wait on hold he took my radio ID, billing address (but not credit card), and phone number, and said he set me up with a free trial. Then in the middle of a sentence, which I thought was going to be "thank you for calling Sirius XM, have a nice day," he hung up or was disconnected.
A day later, still no XM, so I called XM CS again (phone number on web site). After another long wait on hold I got another polite agent. She could find no record of me, neither by name, phone number, nor radio ID, and began taking down all the information again. I asked if she would call me back if she got disconnected before she finished, and she said no, they are not allowed to phone out.
First she said my trial is over (2 weeks!) and said she would set me up a new paid account. I politely repeated again the situation. Then she said my radio didn't exist. I repeated the number, and she found it. She couldn't do anything from her computer and put me on hold to go to another PC to do it. She returned and said that "since I had been such a good long time customer" (two weeks, unpaid, during which they had no record of me?!) she would give me a free trial. I repeated that the car came with music plus traffic, and she said she'd get to that later. She gave me a userid, password, and account number, at which point I became comfortable that if her phone disconnected I would not have to start all over again at zero tomorrow with another CS agent.
Then I verified the trial information with her. What was the period? 3 months. Fine with me. And it's music plus traffic? No. She said they didn't have any free trials of traffic, only music. I said that doesn't do me any good. I'm not very interested in the music. It's the traffic I want to try. She put me on hold again, and when she came back she said I had a 6 month trial of music plus traffic.
Now, if the traffic service turns out to be useful I only have the billing problem. I'm not willing to give them a credit card, given the quality of their "customer service" and reports that they refuse to close accounts and continue to bill without authorization. I could try to send them a check for a year's subscription in advance of the billing date. Or their web site mentions pre-paid cards, though it doesn't say where one might acquire these cards. Or I suppose if all else fails I could pay with a one-time-use credit card (the type meant to guard against Internet fraud) or with a stored value debit card. Has anyone found a good way to pay XM? (I guess I do have about 6 months to figure it out.