You cannot simply replace the head unit for the shaker 1000... they use unique wiring which makes it impossible to just slide another head in there. I personally would have just gone without 500 or 1000 and bought an aftermarket (in fact, that's what I did). Also, we still havent figured out if there is any actual AUX input on any shaker or if the appearance of an aux is just leftover from another very similar model. Without a tape deck , the FM transmitter appears to be the only way. I will say that in a highly populated area, it was very difficult for me when I tried to make use of such a method. There didn't appear to be a single station that was interference free for more than a half hour. I also had trouble if a passenger blocked the line of sight to the antenna.
Just my 2 cents.