When normal A/C troubleshooting procedures fail to turn up anything wrong but the vent temperature is still not as cool as it should be, there could be a problem with one or more temperature sensors.
The computers in our Reattas use temperature to control any number of functions for the engine and body so a malfunctioning sensor could be affecting performance, but not set a trouble code or report an out of range condition in diagnostics. Even the MAF sensor is really a very sophisticated temperature sensor sensing the temperature loss of a heated wire in the air flow.
So how do you know if all these sensors are working correctly?
Just catch them at a time when they should all be reporting ambient temperature to the ECM / BCM and compare the readings. If any of them are more than a degree or two off from the rest you can be suspicious that it is reading correctly.
Perform the following checks after your car has been parked for many hours and out of direct sunlight like early in the AM or while parked overnight in a dark garage.Enter diagnostics mode, ECM Data, and record the temperature readings for:
ED23 Intake Air
Then switch to BCM Data mode and record:
BD21 Coolant again
BD27 AC High Side
BD28 AC Low Side
If any reading is more than a couple of degrees different from the others, I'd check and clean the connector to the sensor and see if it's OK. If an extreme temp limit is displayed (usually -40 or +151) then the wiring from the computer to the sensor is suspect. If these efforts don't bring it back into line the consider replacing it.
The AC temp sensors aren't available new anymore so some used ones from a donor GM vehicle will have to be found. (Hi and Low side is the same part, used in many high end GM cars of the era)
The interior and sunload sensors are under the dash pad and like the photocell, a PITA to get to. The outside temp sensor is low up in front of the radiator I believe. Not sure about availability of these.
The engine side temp sensors should be available at most FLAPS. Intake Air is on the underside of the air filter box, the coolant one high on the drivers side of the block a little to the front of the coolant pipes to the throttle body.