The fan does not turn on when I turn on the a/c or heater in my 1996 saab 900 se

It's most likely the blower motor resistor or relay. My money is on the resistor. It's a pretty common issue on all makes and models of vehicles. I'm not sure where yours is located. Domestic cars and trucks have them on the top of the blower motor housing which is behind the glove box on the passenger side. A few torx bit screws remove the box liner, and a socket removes the couple of nuts holding the resistor on. Other than that, it's just a pigtail wire harness that plugs into it. Pretty easy job. The resistor for your vehicle (96 Saab 900 SE) is $53.99 locally here in Minnesota.

Check fuses or realys