Another thought-from the trim shop that has helped me with the cars is a dab of Mineral Spirits (small can from Home Depot). This stuff is very safe to use on most any part of the car so far as I can tell. It works great to remove road tar and other marks on the paint. The shop told me to never use stuff like Armor-All on the dash just this. You could try a Q-tip coatedf with a bit of denatured (Home Depot) alcohol. Remember to use a wet towel to cleanup as you go.

The last option I can think of is one of the dash covers-they are really good at protecting the surface from the Sun.