My previous auto car (Mitsubishi FTO) would automatically change down a gear if it started to runaway with itself down a hill, etc. In fact, as it was a tiptronic "sports" car, it actively encouraged you to drop a gear to lose some speed.
AFAIK there is no facsia adaptor for the RHD MK1 GS300 so you'll have to either make one yourself or have one made. A few people on here have done this..
I adapted the wiring to run an aftermarket amp from the standard headunit which was fairly simple.
It's ten inch and I'm pretty sure it's made by Pioneer
EDIT: Pioneer 88160-3A12D, 2 Ohm. Doesn't mention power rating.
If you're after one I've got a fully working one sat around.