Would a bad igintion switch cause the cruise control not to work and the hvac to malfunction? The hvac will sometimes work. as in the display unit. just replaced the control module so its not that. The cruis control also does not work. Could this be an ignition switch?

Take your Rainier out for a drive and see if the cruise control still works. If it does, the VSS is likely fine. If not, the VSS could be the culprit (Vehicle Speed Sensor). Cruise control also depends on throttle position. Ensure that the throttle body and related components are functioning properly. Examine the wiring going to and from the VSS. Look for any damaged or frayed wires.

The most common reasons for cruise control malfunction are failed control modules, faulty sensors, or switch issues. While you've already replaced the brake switch module, it's essential to verify that it's functioning correctly.

Additionally, check other control modules related to the cruise control system. While you've already replaced the brake switch module, there might be a separate cruise control module. Inspect this module for any signs of malfunction.

Powertrain Control Module (PCM): The PCM monitors the signal circuits of the cruise control switches to determine when to capture and maintain the selected vehicle speed. It uses the throttle actuator control (TAC) module motor to control the vehicle speed. That would be something to check. Visually check cable connections and for corrosion. A multimeter can check the battery which can affect the PCM.

Sometimes, disconnecting the battery to clear any fault codes can resolve cruise control issues. Possibly that might work even though you see no fault codes.

which make of car and year search finds you the part

are you up to DIY and purchase of parts fitting new parts

its sensors near the top of the brake pedal under the dash

P0571 Cruise Control/Brake Switch A Circuit Malfunction
P0573 - Cruise Control/Brake Switch A Circuit High

C107C - Brake Pedal swithch 1/2 Stuck
C107D -brake pedal switch 1/2 correlation

some examples of locations cruise/brake switch

code p 0112 refers to intake air temp (IAT) sensor --circuit malfunction===cause --wiring short to graound--wiring short to positive --IAT sensor ---ECM
code p 0117 refers to engine coolant temp (ECT) sensor--circuit malfunction=== causes --coolant thermostat---wiring short to ground--wiring short to positive ---ECT sensor
code p 0122 refers to throttle position (TP) sensor A/ accelerator pedal position (APP) sensor A low input === causes wiring short to ground--TP/APP sensor --ECM
There are 3 responses to p 0122 and all refer to TP switch /sensor plus wiring open /short to ground/ short to positive
code p 0513 refers to incorrect immobilizer key=== causes immobilizer system
code p 0556 refers to brake booster pressure sensor---circuit range / performance=== causes --wiring---poor connection---brake booster pressure sensor---ECM
code p 0565 refers to cruise control master switch. off signal --malfunction === causes wiring---cruise control master switch--ECM
by reading all the codes you will see that common causes are wiring problems and ECM so may be a quick check would be to replace the ECM with a known working unit first and work back from there

If cruise control main light comes on but you cannot set a speed (no set), you could have a bad brake pedal switch.
(If the cruise control light comes on, chances are that it is NOT a fuse issue).
Some vehicles have two switches (one for brake lights and one for cruise control cancel) and others have a multifunctional, single switch.
The function to cancel cruise control is very sensitive so it only takes a small movement of the pedal to cancel cruise control.
You can try lifting on the brake pedal while trying to set the cruise, but this does not alway work. Replace switches are pretty cheap (less than $30) and pretty easy and quick to replace.

1. Disconnected or damaged vacuum lines to main cruise control unit;
2. Bad or misadjusted brake light switch (most likely);
3. Defective cruise control switch;
4. Failed main cruise control unit.

• Brake Switch (Cruise Control) Clutch Switch
  
• Cruise Control Module
• Cruise Control Servo Cruise Control Switch
  
• Release Valve
• Vacuum Pump
• Vacuum Vent ValveVacuum System, Checking For Leaks
  • Remove vacuum hose from vacuum pump seal off.
  • Remove lower instrument panel cover, left.
  • Push servo unit diaphragm in, to do this press brake pedal down by hand until pressure point.
  • Then release brake pedal.
  The diaphragm must not move.
  
  NOTE: Malfunctions can be caused by an incorrectly adjusted vent valve, leaking servo unit or through cracks in the vacuum hoses.Here is a quick run down of the components I need more info to help u more is ur car a syncro or a sedan is ur engine a 1.8, 1.9 or 2.8
  

• Brake Switch (Cruise Control) Clutch Switch [img src="http://i.fixya.net/uploads/images/no-p.gif">
• Cruise Control Module
• Cruise Control Servo Cruise Control Switch
• Release Valve
• Vacuum Pump
• Vacuum Vent ValveVacuum System, Checking For Leaks
  Remove vacuum hose from vacuum pump seal off.
  • Remove lower instrument panel cover, left.
  • Push servo unit diaphragm in, to do this press brake pedal down by hand until pressure point.
  • Then release brake pedal.
  The diaphragm must not move.
  
  NOTE: Malfunctions can be caused by an incorrectly adjusted vent valve, leaking servo unit or through cracks in the vacuum hoses.Here is a quick run down of the components I need more info to help u more is ur car a syncro or a sedan is ur engine a 1.8, 1.9 or 2.8

I can't diagnose unless you can do some testing? This is part of diagram for automatic hvac. At the blower motor, terminal B is voltage, terminal A is ground. Use a test light, check voltage and ground with hvac on, Have to check when problem is ongoing. If good voltage and ground with no blower action, suspect the blower motor is faulty. Of course, if your missing voltage or ground, have to track that circuit. Faulty blower motors may run intermittently. When problem is ongoing, hvac on, you can try tapping blower motor with screwdriver handle see if it starts running, if it does, I suspect the blower motor has a problem. Check hvac trouble codes, where applicable.

One thing that may cause this is a bad tail/brake light socket or bulb, a loose conection, or bad ground.

When looking at a power distribution schematic, the HVAC and cruise control share a dedicated feed from the ignition switch. It sounds to me like the switch has an intermittant fault, since the display is lit and then when cranked it goes dead (as it should) but the switch may never remake the contact when returning to run causing the loss of power.