Wow! I tested out the adapter tonight and it sounds way better than the modulator even though I was only getting sound from the left channel due to cable adapters from 1/8" to RCA (my fault). I'm going to go get a new cable this weekend and get this thing installed next week. I figure I might as well leave the FM Modulator in there and connect the cassette adapter to the back channels.

Thanks again for the great tip!
Matt