best way i found is get a kensington FM transmitor, ie 1 of these
http://store.apple.com/1-800-MY-APPL...nplm=TD145LL/A
they are cheaper else where online and it DOES involve buying something HOWEVER i have found sound quality to be on the level of my hispeed ipod cable I have for my alpine HU that i use in car. ALSO this doubles as a car charger and works awesome. i rarely get interference. only thing i don't like is sometimes when i'm playin the ipod and the car was in acc and i turn the car on, i have to unplug the iphone and plug it back in, else it doesn't work (sometimes)