This is a typical GM issue. There is either a small box from the fan switch going to the blower motor or it is in the switch assembly (not sure how the 96 Cav's are set up)
But there are 3 resistors hooked to the switch. The resistors are different values for each of the 1, 2, and 3 speeds. The 4 (high) has no resistor. As they get old and such the resistor crack and you lose that speed of the fan. The reason 4 doesn't is there is no resistor just a wire.
Pull the switch assembly out of the dash. If there are 4 wires out of the switch, trace it to the little box. You can read the resistor values and buy them at Radio Shack or other electronics store. It would cost you about $2.00. If it's just one wire to the fan, You will need to take the switch assembly apart.
If I recall speed 1 is about 1.5 ohms, speed 2 is about 0.7 ohms and speed 3 is about 0.2 ohms.
Typically (GM likes to use the same color codes for almost all vehicles) on the back of the blower switch if there is a seperate blower resistor module, You should have a yellow (speed1), Tan (speed 2), and a Light Blue (speed 3) wire coming out of the switch. Follow these to the module. The orange wire is the high (speed 4) and should just go to a seperate relay.
Another thing is many vehicles use the same module so if you want to just grab one from a junkyard, Just remember to take an ohm meter with to verify it is good.
Hope this helps.
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