I also have an aftermarket Kenwood. The problem I have with it is, it is very hard to control the iPod through the deck and once it's plugged in, you can't control it from the iPod itself. IMO it only works well if you just put it on random and let it play. BTW the model I have is an excelon KDC-X789.
Also, the link I posted up earlier for the FM Modulator works pretty well. The doesn't use a transmitter like most do, it connects directly to the FM antenna in the back of your radio. Then the antenna plugs into this device. When turned on, it blocks the signal from the antenna and puts in it's own signal. That way you don't get any interference because it's a direct connection.