Does your radio have a Sirius/XM tuner, or is it "sirius ready" or something like that? "Ready" means that you can attach an external tuner that will be controlled by the head unit. If you have the tuner, try looking here:
https://www.siriusxm.com/activationhelp . There also a reasonable chance the ESN will be on a sticker on the tuner module, or on the head unit case if the tuner is built in to the head unit.