From a technical perspective, the capability is certainly there. If the vehicle has a cellular modem built in (a large % of new cars do), the activate/deactivate instruction can be sent that way. If it doesn't, the SiriusXM satellite system has a control channel to which all the radios are "listening," which is sending activate/deactivate codes on an ongoing basis, so it wouldn't be technically that hard for a large customer like Hertz to be able to send radio IDs to be included in that feed for activation or deactivation.

My hunch, though, is that there's only action one way. If you say you want satellite radio, and the radio isn't active, the activation signal is sent, but if you say no, they aren't shutting down active radios. Not worth the effort.