Yesterday I was making a booking on booking.com, and even before I finished it offered me a 4-month free subscription to the Sirius XM app. This is not the car-connected subscription, but you can certainly play the app in your car. This might have been because I'm a former Sirius customer, or because I'm a booking Genius (blush) or maybe because I use the same email address on both.
There's also a $1 for 3 months promo on the website now, and that's for car AND app. If this turns out to be a "new customer" thing, remember you can put dots anywhere in your email address, and most websites will think it's not the same. E.g.,
abcd.efg@gmail.com and
a.b.c.d.e.f.g@gmail.com are the same as
abcdefg@gmail.com to the email system, but different for most website registration purposes. Some will also accept the
abcdefg+sirius@gmail.com form, but I think Sirius doesn't.