I did measure up where to put the back ones ages ago, but don't know where the notes are now.
You'll find that there are cutouts in the polystyrene behind the rear bumper, this is where the sensors are positioned.
Note when removing the rear bumper: it looks like an upper and lower half, but it's actually all one piece! Get a big sheet of cardboard to put it down on, you don't want to scratch it on the tarmac!
If the sensors are like mine, you'll need a 20mm holesaw, plus a triangular file to cut the keyway at the top (it's easier than drilling them).
I should be able to email you the instructions for fitting the LPA400 (what I have) to an IS200/300, you may find it useful (except for the measurements of course!).
PM if you want them.