If it is a mk1 GS300, the amp is in the boot under the CD multichanger. Easy to change out and doesn't have to be linked to the ecu or headunit. If it is a mk2 GS300, the amp is next to the glove box and just needs the glove box removing and the under dash trim. Again, an easy job to do and no tie in to the ecu.
There are some details on aftermarket installs in my signature block below, not the easiest thing to do, but not too bad.