Thanks for the photos. I did check behind the glove box and there's nothing like the photos. Just two plugs/connectors screwed the to the body that are facing the right direction to be pluged into what your photos show.
I'm at work now and have successfully put the Pioneer system in so can't check the unit number but will when I get home. In my IS200 I wired the Pioneer unit in, without bypassing any amp, by connecting the Pioneer unit "Mute" wire to what appeared to be the built in amp wire - it worked.
In the IS300 I tried the same and flattened my battery. So I have now wired the Pioneer unit "Power Amp" lead into what appears to be the built in amp lead and its working. No flat battery or sound issues. But still no amp behind the glove box. I suspect there isn't one although there must be otherwise why the built in amp wire to the Lexus head unit - which I have now wired to the Pioneer "Power Amp" lead?
In my searching on the web I did pick up blog where someone stated they can be behind the glove box, under the front passenger seat or built into the boot. I cannot find anything in the other two locations and can't find photos anywhere so do not even know if I'm looking in the right places or behind the right panels.
I can therefore only presume one of two things - 1. there isn't one and the built in amp lead does draw power if connected wrong (even if there is no amp) or 2. it is located somewhere else and I just managed to wire it up right thanks to the Pioneer unit having a "Power Amp" lead also thereby providing the same connections as the Lexus head unit?
I'll check on the Lexus head unit number and reply once I'm home, as for the car model in SA they have two models of the 200, 300 and 250 - the standard and the SE. The SE just comes with extras like sun roof, both front power seats, arm rest, better radio, etc. Otherwise they are mechanically the same as the standard models.
My IS300 stereo head unit number is 13910.