Hi, there isn't any easy way of changing the specific resolution, though its possibly that your in car entertainment system only accepts one of either PAL or NTSC video inputs.

If you go to 'Settings' from the menu on the N900 and scroll down until you get to 'TV out'. Tap on 'TV out' and change the output format to whichever one is not selected at the moment - ie if it currently says PAL change it to NTSC and vice versa.

Then try reconnecting to the in car entertainment system. Hopefully this works, if not let me know and I'll do some digging.