easiest will be what shelbyautumn said; by an FM transmitor that plugs into the headphone jack of your iPod and lets you listen on your car stereo. I had one back in the day... cost <$20 and worked well so long as there wasn't an FM station on the same frequency. Sound was decent; about waht you'd expect from FM radio.
Alternatively, I'd make *sure* you have no aux connection into your car radio. Most CD players still had this option, even 15 years ago. Often it's in the back (i.e. inaccessable) on older models. If that's the case what it will take is a $5 and a few minutes running the wire - a good place is into your glove box. Google your model number and/or look carefully for the words "aux" printed (often it shares space with another button/dual function).
You can get a CD player cleaner which might help, but sometimes it's just the quality/condition of the CD player itself. The cleaner-disks cost about $6-10 and have tiny brushes which are supposed to wipe off dust from the laser head. It can't hurt to try (other than costing you a few bucks).
FInal option is just to replace the head unit (receiver). You can find new units with a dedicated jack for ipod/smartphone for about $45, but you can also find used ones for almost nothing. You could start by asking car installation places what stereos they have replaced they otherwise can't re-sell, or ask bodyshops. A buddy of mine had his car stereo stolen and he found a basic replacement for $10 that had a CD player and a ipod plugin.
