depends on the car really.

almost all new cars use canbus or similar datalink technology to link the main ECU to dash, airbag units and various other sensors etc around the car.

it is common place for the dashboard to shit a load of fault codes/warning lights if the original ecu is removed or has loads of wires cut. Also many features like speedo, rpm, water temp, fuel guage are fed through the ECU and on the datalink so you can have problems with getting the dash to work full stop.

changes started happening mid to late 90's and came in gradually over the next few years depending on car and manufacturer.

Youll need someone in the know on saabs to find out for your car.

You can work it out in some cases from the wiring diagrams.