This is a script I recently made for cruise controling, press F6 to toggle the cruiser on/off. I have only limitted the max speed, if you want to modify it and add an acceleration or transmitter limiter your choice.
See you around.
EDIT: Removed the broken Mediafire link and added a dropbox link
EDIT2: Added 1 more constraint, you need to be the driver to be able to use F6 (Not tested tho, no one tested it with me)