Heater/ac switch speeds 1 and 2 don not work in 99 Silverado

The brown wire at the speed switch is the power wire. Powers is then directed to the blower motor resistor throught the orange, lt blue and yellows wires. The blower motor receives power through the orange wire. Check for power at the fuse with the key on. If there is no power the problem could be the ignition switch or the wiring in between the switch and the fuse block

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there is a relay in heater box go to autozone they can get you one and let you know where it is located

SOUNDS LIKE THE SWITCH & OR THE BLOWER MOTOR RESISTOR, ARE THE PROBLEM WITH THE LOWER SPEEDS. SPEED 5 MAY HAVE ITS OWN FUSE, REFER TO OWNERS MANUAL FOR INFORMATION ABOUT THE HEATER/AC FUSE.

most probably the heater resistor is bad , its just beside the blower motor, with a harness of 4 to 6 wires connected to it , usually blown because the blower motor is drawing too much power

You have to take apart the dash. If its the 99 up style. If it is the classic style (96-98 dash) you don't.

Purchase a new mode selector switch for it at any Chevy dealerships parts department

It is a 99% chance you need a new fan speed resistor board.
It is beside the fan motor.... it sits in the heater/AC duct work to use the air flow to cool itself.
It costs about $35 and takes 15 mins to change.
Did you loose one speed then another and now only high works?
That's the usual way they fail.

blower resistor.. located under the pass. side of the heater case ..

your speed switch located in heater box usually one wire to it in passenger feet area above "TOES" usually you loose 1 or 2 speeds on fan but it is unplugged or completely cooked good luck!

there is no relay for the heater fan it's a 15 amp fuse and is on the accessories circuit. the reostat is responsible for all 3 speeds .with the key off there should be no voltage to the blower motor whether the fans speed fan switch is on or off. the fuse will be located in the fuse paneleither on the drivers left side in the dash or underneath the dash in the fuse block. you have a short in the harness that is under the dash and most likely it's a bad plug on the harness. your going to have to do some searching but that's where the problem short is... please rate