If I'm not using the antenna originally mounted on the car for anything else, can a Sirius radio use it with some type of adapter?

I really dont want to use the one that comes with it, PITA to install on the outside of my car.

satellite radio frequencies are way above those used for regular am/fm broadcast. The little mag mount sat. radio antennas are powered/amplified antennas made to receive satellite frequencies. Take 30 minutes and run the antenna cable. Depending on your vehicle, it can be anything from easy to very frustrating, but you only do it once.

are you speaking of an OEM XM or Sirius roof mount antenna that you want to adapt aftermarket equipment to?

what make/model/year are you asking about?