Well, I have already installed FM RDS Notify, to play music over FM. So you'll tune in and store Maemo FM at whatever frequency you set it to.

If this works like that and you're not streaming music from the N900, you'll probably have to set your stereo to FM and pick your stored Maemo FM frequency.

Now, where the N900 differs from most other cell phones, is that you can send audio AND TEXT to your non-bluetooth radio. Basically pretty much every old car stereo. Unfortunately it'll have to work within the FM/RDS limitations - unlike bluetooth handsfree it'll not take control and pause your CD.