Hi Trucker
Be careful - if you don't match the impedance of the speakers correctly to your amplifier (in this case, built into the cd/radio I would've thought), it'll damage your amp.
If you've got a spare pair of terminals it should be fine to add another pair of speakers - but check with your local dealer (or even the local car-radio shop) about the impedance your radio can cope with.
If you match your speakers carefully, it actually makes the amp work more efficiently and therefore louder! I'm sure someone on here has the technical speak to put that into words!
Enjoy!
Mac