How can I fit cruise control unit to Rolls-Royce Silver Shadow series 1 1974?

There is no cut and dried answer to this question. You will want to make a separate electrical circuit with a fuse as you will cause problems if you tap into one on this car. You will need to purchase a universal kit and make this fit your car. Some of these are controlled electrically and probably by computer, although others will use engine vacuum. There will be at least one mechanical actuator to operate the accelerator cable and maybe some kind of speed sensor to keep the car at a constant rate. This involves using two speedometer cables to hook that up. You will also need a linkage for the throttle, places to run the wring through the firewall and a means to keep said wiring looking tidy and neat (and to keep it away from hot or moving engine parts). You will also need a handy area to locate the switch (away from the ignition switch which is on the left side of the steering wheel). However, for safetys sake, this will need to be on the left side as well. You will also need a switch to cut off the cruise control that operates when the brakes are applied as another safety feature. As you might have guessed, this is a job best left to a professional. Any money you save doing this yourself could mean putting yourself and others at risk. If you decide to do this, follow the directions on the kit and be prepared for adapting parts to fit. This is not an easy job.