I successfully completed the Aux In mod for my 99 RX 300 using the Aux In for the IS300 instructions.
There are a few things about this you need to know before doing it.
You need to be in CD mode. If you have a disc in the magazine, it will continue playing the CD.. you just won't here it. If you put in a blank magazine, you'll see ERR2 on the screen and the CD motor will not spin.. This is what I do to save the CD motor.
If the iPod is playing and you switch to another mode..FM/AM/Tape..you'll here both sources at once.. simply pause the iPod.
The IS300 mod involves slicing into the wires coming from the head unit so you need to switch to go from FM/CD/iPod..
I find it more convienient to not have to do this and simply switch to CD mode and turn on the iPod.
I spliced into the CD changer Line Out wires and put in the socket and relay. I put the switch under the glove box since I rarely use the CD Changer so I didn't want to have the switch visible in everyday use.
The instructions I followed are here:
I can only upload 1 picture at a time so there are more pictures in the replies.
Let me know if you have any questions or need more clarity.
When you remove the glove box (2 bolts at the top left and right when the box is open and 2 at the bottom left and right when the box is close), you'll see where the wires come in on the right. This area has a lot of space so I put the socket and relay here.