Clutch and Brake Pedal Switches:
The clutch and brake pedal switches work the same way, but the clutch pedal switch is the one that normally needs adjustment as the clutch wears down. It only needs this on older cars, where the clutch pedal spring is too worn to return the pedal completely to the rest position (against the clutch travel adjustment screw).
The pedal switch works correctly for the life of the car, but only if the clutch cable, clutch return spring, and pedal travel adjustment screw are all working correctly together.
When new, the clutch pedal returns to its rest position, first due to strong tension in the clutch cable, and at the end, through weaker tension in the clutch return spring. As the clutch wears, the clutch cable mechanism is supposed to adjust one click at a time, and the pedal return spring does not have to do much work (2-3 millimeters at most).
When very worn down, or when the clutch cable is old, the cable adjustment does not work 100%. As the clutch wears and the clutch cable gets out of adjustment, the clutch return spring has to do more of the work, and as the spring gets "tired", the pedal does not return all the way against the stop at the end of the clutch travel adjustment screw. In that case the switch does not close when the clutch pedal is released.
One can test this by pulling up at the pedal lightly
by hand (with car parked and engine off), or tip of one's shoe while driving at a steady speed. If there is play in the travel of the pedal, the switch may well close when pedal is pulled up against the rest, but open again when the pedal is released, and stay open.
If the switch stays open with the clutch pedal released (foot off the pedal), the cruise control will not work.
The right (and expensive) solution would be to make sure that the clutch cable is working correctly, or replace it with a new one, but the switch can also be made to work by pulling the switch plunger out manually, re-installing the switch, and letting the plunger ratchet reset itself to a new rest position.
The ratchet only resets itself in one direction, because it is designed to do this once, on installation, and not gradually a click at a time (like the clutch cable adjustment).
So, when someone says that the switch is not adjustable, strictly speaking that is true, because it adjusts itself only once, when installed (or re-installed after clutch cable replacement).
Sometimes the plunger can be pulled out with the switch in place, but often not. The problem can also be solved temporarily by taping something the size of one or two stacked nickels to the clutch pedal where the pedal pushes against the switch. This can be easier than removing the switch which is usually fragile.
With enough play, the switch can break where attached to the metal bracket. This is the only time it should need to be replaced, but mechanics often replace the switch instead of resetting the plunger, because the part is cheap compared to shop time, and if you pay over $100 to get the cruise to work again, you may as well end up with a brand new switch with a plastic clip which is not fragile due to age.
Some bad attempts at fixes:
Unfortunately, some people have bought used cars where the clutch pedal switch was missing, and/or bypassed), instead of replaced. This eliminates one of the safety interrupts on the cruise control (the other being the brake pedal), and hard to find for a new owner.
Previous owners of mechanics may also have changed the clutch pedal travel adjustment in an attempt to fix the problem. This can seem to work, but it is a REALLY bad idea, because it can interfere with the correct function of the clutch. (Clutch pedal travel is set once in the factory, and may sometimes need to be changed slightly when a new clutch is installed In other words, don't mess with this unless replacing a clutch, AND you know exactly what you are doing).