2) ... If that's true try charging the phone while the phone is off (off, not just asleep). This will help to determine if the interference is coming from the phone or the charger.
If the answer to mhotovec's step #2 is "yes, it only is a problem when the Eris is charging with the phone
turned on", one way to mitigate the problem is with one of those snap-on ferrite chokes on the charger cable at the end closest to the Eris. (That's the little lump on your laptop's charger cable near the plug for the PC). Snap-on types can be found through specialty electronics distributors, but be advised that they are designed to suppress EMI in various frequency ranges - if you can find one that is designed for FM suppression, so much the better.
Another way might be to simply move the charger to a different plug to get the charger cable a little bit away from the radio - if your vehicle has back-seat charger plugs, give that a try (if it is convenient). This would work only if the interference is "over the air", so to speak.
There are lots of ways that RF can interfere - direct radiation is one way, but conduction along power mains is another, and it is possible that the charger plug and radio are on the same fuse circuit in the vehicle. This is along the lines of #5 in mhotovec's suggestions. In-line or choke-style ferrite filters on the radio's power supply line are another possibility - or even moving the radio to a different circuit. (Depends on exact details of your car's wiring harness).
Probably you are in for a little experimentation to narrow down to a solution - let us know what you find out. (Also, it might be a good idea to post which charger cable you are using - the Verizon car charger?)
eu1