I posted a lengthy post from my phone that got lost...so in short:

I was in a family members new (used) car and there was a sat button. They seemed interested but probably didn't know how to activate it.

Since most used car buyers are going to be under 30 (IMO) then a great place to advertise to them would be on the internet.

A great way would be one of the ad windows on the side of any website that you can click on titled "how to activate the Satellite radio in your new (used) car".