It seems, from the link below, that unfortunately Car Talk is only available in Real Player format for streaming and in Audible format for podcasts, which means neither will work with TiVo.
http://www.cartalk.com/Radio/Show/
http://www.cartalk.com/Radio/Show/audible.html
Shame too, I listen to these guys all the time.
There are more convoluted ways to get the audio to TiVo, such as shoutcasting everything that goes through your pc's soundcard to the TiVo(including various beeps, alerts, etc.).