Check the basics: is the compressor cycling? If not, is the fuse blown? Otherwise the problem could be any number of things. AC troubleshooting is, like most troubleshooting, a detailed but logical process.
You have no cooling (the SAAB uses a full coolant flow heater and an air blend system to mix cooler air with heated air to reach the set temperature, ther's no coolant flow control valve in the heater) but the rest of the system is working. So, it has to be the refrigerant is not flowing through the evaporator matrix (9in the heater intake in the aquarium area). The most obvious reason would be the compressor isn't working. It operates on an electric clutch on the pulley, which runs through a fuse and a relay. When the compressor cuts in and out the pulley keeps turning but the compressor drive stops, also a loud click can be heard as the clutch engages or disengages.
Next is the possibility some dumb *** emptied the refrigerant and forgot to re charge it. Next is the very common problem of a seized expansion valve in the AC system. A failed hi/lo pressure switch could be it. Also, the anti frost switch sometimes fails.