tatersalad:
Are you sure the factory wiring isn't already there? could you just add the factory controls to the system? lots of the newer vehicles are all the same harness just without the buttons. My truck is completely wired for every extra switch that they make and its as simple and plugging the factory switch in. I wouldn't try to home brew that one man... its not worth your life or someone else.
this is probably your best bet. there is a very high probability that the wiring harness are made all the same, it might lack a module under the hood and whatever buttons on your steering wheel. can arduino do this? probably, but this actually seems like a situation where you actually SHOULDNT use arduino. ill pass over the 'thousands of [pick a unit]s moving at very high speeds' saftey thing, and go straight to saying even somebody with a lot of experience with arduinos would probably not use an arduino for this. its just too much work compared to using purpose made components. this becomes MORE true with newer cars which are more and more controlled electronically. just touch the wrong wire and you might trigger your anti-thieft, disabling ignition until the dealership resets it. i know it can be difficult to ask for help with something and get told nothing but 'dont do that', so i hope i can convince you that you dont really want to.
oh, another important point: cruise control is more than keeping the accelerator in one spot. not only do cars often have a special gear or something with the transmission specifically for CC, they use feedback to pickup or back off the gas slightly as needed, and is an early example of PID controllers, which is probably not something you can impliment with a potentiometer