1987 Toyota Supra Stereo Wiring (some of these colours are the same for other MKIII Supras, others are not, contact me if you have a specific question). For 1990 Wiring see here.

There are two plugs that connect to the stock stereo and the pin-outs are shown below. Unfortunately the manual doesn't have the plug layouts, just the colours and purposes.

Plug A
Pin 1 - Light Green - Front Right Speaker
Pin 2 - Pink - Front Left Speaker
Pin 3 - Grey - Accessory Power (i.e. on when key is turned to ACC)
Pin 4 - Black and Red - Antenna Control Relay
Pin 5 - Blue - Front Right Speaker
Pin 6 - Violet - Front Left Speaker
Pin 7 - Not Used
Pin 8 - Pink and Blue - Antenna Control Relay (unsure of purpose)
Pin 9 - Brown - Earth

Plug B
Pin 1 - Red - Rear Right Speaker
Pin 2 - Black - Rear Left Speaker
Pin 3 - White - Rear Right Speaker
Pin 4 - Blue and Yellow - Permanent Power
Pin 5 - Yellow - Rear Left Speaker