Do you know how to change the FM modulator's transmitting frequency? It simply sounds like the FM frequency currently being used is competing with a local FM radio station. Moving the modulator's transmission frequency should do the trick.

I don't have the an Invision system (yet) so I can't help you, but check your instructions to see if you can change the frequency. It's often done by moving small DIP switches that I imagine would be located on the control box (or you may have to remove the cover). You'll need to find a frequency that is clean (you don't want even faint radio noise if you can avoid it). Even though you had a dealer install it, you should learn to do this yourself in case you move of if a new radio station springs up on your transmission frequency.

Have you tried just listenting to 88.3 on the car radio or a home stereo? Is it a "clean" frequency as I've described above?