Fault code c1

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.

Here is the fuse box diagrams for the 2 different engine options...and the 2 different box locations
Buick Rainer 2003 2006 fuse box diagram Auto Genius
HOWEVER ... I do not think there is an associated fuse for cruise control. That function is totally run from your on board computer (ECM). What can make the cruise not work is if any fault codes have occurred... Has your check engine light come on?
Some people with the same problem (that do not have an OBDll scanner to read or clear codes) disconnected their battery to reset the ECM. The cruise control then worked .... but if the root cause of the fault is not fixed ... then the same thing will likely happen again
Helpful?
Good luck with your troubleshooting

It seems the pcm is detected a fault with the engine. Even if the problem goes away , the vehicle will have to go three drive cycles without engine faults before the cruise control use will be allowed by pcm.

run the fault codes and look for a faulty VSS sensor as it works the speedo and gives information to the cruise control as to road speed to keep the vehivlr at the set speed

run the fault codes
cruise control needs the following to work together as one or more will stop it from working.
VSS sensor, (road speed /speedo)
engine rpms/tacho
brake switch sensor/
clutch switch sensor
cruise control switch

You can't adjust the codes. When the cruise control is working properly again, the engine light will stay out once it is reset.

The problem could be caused by several things. Did you check to see that you actually have vacuum at the cruise control servo? Or did you just "look" at the hoses?

Is the cable connected to the throttle body like it is supposed to be?

Have you scanned your PCM to see if there are any fault codes? Is the check engine light on? The PCM will not allow cruise control engagement if there are any fault codes present - including some codes that are not related to the cruise control system itself.

The speedometer is electronic and controlled by a micro processor, the needle is operated by a special DC electric motor called a "stepper" motor, this little motor can cause this problem as well as the Vehicle speed sensor on the transmission (see photo below), either part can cause the problem, the cruise not working would lead me to believe the speed sensor is defective, but you should have a Check Engine light on and a fault code for the sensor, do you have either? Autozone and others do the fault code test for free, ask for an OBD2 fault test. Here is the picture of the sensor I mentioned.

Actually FIXING the cruise control is usually not very hard. The hard part is finding out what is WRONG with the cruise control. Cruise control problems are usually caused by a broken clockspring (or contact assembly) in the steering column or a faulty cruise control servo assembly. Of course, there are several other possible reasons for the cruise to malfunction, but these are the most common.

The first step would be to scan your Engine Control Module (ECM) for fault codes. Please note that cruise control faults will not normally cause the Check Engine light to illuminate, so it is NOT safe to assume there are no codes present just because the light is not on. If the Check Engine light IS on, then you need to scan your ECM anyway! If there is a problem with the servo assembly, there will most likely be a code set. Problems with the clockspring will not usually set a code. A good clue that this could possibly be the problem is an inoperative horn and/or the Airbag warning light being on. (A broken clockspring will also cause these things)

Do you have the Check Engine light on? If there is a fault code set, the cruise control will not work. You need to have the car checked for fault codes. Take it to Autozone or do it yourself if you have a code reader.