Should be inside the airbox, it operates a flapper on the air input to
that metal hose over to the exhaust manifold. In a mild climate you can
just remove that hose and no longer risk killing expensive AMMs, the
preheat system is worthless, especially on a turbo where you cool the
air right back down again with the intercooler.

Are you the original owner? A lot of people take out the thermostat and cap off the preheat hose.
When the thermostat goes bad it tends to fail in the open position, which directs only hot air from
the preheat hose into the AMM (and ends up killing it).

Turbos don't have the hot air system after the 1989 model year, so they
sold you a part your car never had.