Ok, I spoke to Jeff Lange (human Supra encyclopedia... it's creepy). There was an upgraded sound system with CD player available for the '88 year model. The external amplifier mounted right under the stereo (not under the seat). The '89 and later had an upgrade available which had an amp that sat under the seat. There were no packages available from the factory which had an external amp (under the deck or under the seat) in '86.5 or '87 model years.... the harness didn't even have the plugs so the under the seat amp is unlikely to have been added by any dealer as an option.
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