You actually have one other option:

http://www.peripheral-aamp.com/periph_products/cd_changer_autos/gm.htm

The only catch is these adaptors REQUIRE a cd changer for the aux. RCA inputs to work with a GM headunit. The cost for everything is probably between $200-400US depending on the cd changer you want. The positive is that the sound is excellent. If you want to go that route I can help you out. The FM modulator is slightly quicker (not really easier) to install but the sound will not be as good. The cost is obviously cheaper at approx. $60-70US.