I have used SEM products and they are outstanding. The results are excellent and more importantly, they have held up on parts dyed 15 years ago **It is critical that you get the SEM Vinyl Prep which will work best for your dash. (SEM also offers a Plastic Prep, you will want the Vinyl Prep).
My process would be to spray the dash with Simple Green cleaner and using a new plastic brush (like you would use on dishes in the kitchen) scrub the dash. Rinse off, dry the dash and repeat. I would spray, scrub and rinse the dash with Simple Green at least three times. Allow the dash to thoroughly dry for at least a day.
On the day your are ready to apply the SEM, put on a pair of latex gloves to keep the oils from your hands off the dash. Apply the SEM Vinyl Prep per instructions. Wipe off with paper towel. If there are signs of any dirt on the paper towel, give another application of the Vinyl Prep. Once your are satisfied the dash is completely clean, apply the SEM dye in the color of your choice.
Check the SEM website for locations that sell their products. I found mine at the local auto paint supplier.
Best of luck with your project. If you get the dash spotlessly clean and use the Prep, you should have excellent results.
Jeff