Where is the switch typically located? Is it like modern Toyotas where the joystick sticks out the side of the steering wheel?
The switch itself is relatively easy (but it involves removing the airbag so care must be taken), and a little bit of drilling.
The ****ty part is going to be installing the solenoid and bracket to the throttle body linkage. I'd also look at the wiring for the solenoid and see where it goes into. If it goes into a separate computer, sweet! If it goes into the PCM, you may have to steal that as well (in the '05+ Toyotas, the cruise control was build into all PCMs and you simply needed to add the switch to have cruise since they are drive-by-wire, the '04 and below are more complex).