Had to be one of those two.
Mono-motronic could be a bit harder then, as it should have an optical position sensor inside the actuator for fine tuning.
The RPM method is the simplest if you want to make a "universal" type cruise for a manual transmission gasoline engine. A ignition coil is always there and the signal can be taken from that. Electronic speed sensors don't exist on all vehicles. But keep in mind, the coil signal will have to be well filtered before entering the arduino.
The clutch switch is not absolutely necessary. But you do need the rpm signal for the safety of your engine. The clutch signal can be "added" in software. As the car is moving in high gear and the cruise is used, the momentum of the car keeps the revs pretty stable over a short period of time. As the clutch is pressed, that momentum disconnects and causes rpm to shoot up (cruise is still giving throttle). A clever piece of code could notice that and disable the cruise in a blink of an eye... It also takes care of other bad things that could happen (gear pops out, wheel slipping on ice, etc.). Brakes do need a switch though.
What comes to the magnetic clutch, its very simple really. The gears are connected to the cable "spool" with magnetic coupling. The spool is in a basket with a magnet on the side. When the electromagnet is on, the basket and spool move with each other. When its off, the spool will be free to move. So if power to the cruise goes off, cruise goes off.. Though a stepper motor will do this too, when used without a worm gear. Steppers are more accurate as well. They all have their advantages. You have to choose.
Also when making the PID, make it adjustable by serial commands, and have a friend with a laptop handy. Saves a LOT of time and most importantly, gas.
When it comes to electronic throttle cars, messing with the throttle signal voltage is your pick. Throttle contol voltage goes in the controller and modified voltage comes out. Input=output, until "cruise", when output stays at the last value. This is of course risky at best, letting an external controller control the throttle fully, so its done at your own risk.