I have seen reference in the 91 shop manual, but not the 89. Dunno 'bout the 90, and the 91 manual is not great. The 91 had rear air, but no rear heat. There was in the 91 and most all other dual air setups only one compressor, with lines branching to the two evaporators and with only one condesor. I think the 91 had a electro solenoid valve to control refrigerant flow to the rear evaporator.

Not responsible for advice not taken...