That is a common problem with S-10's. I have battled it in the past myself. Make sure you have resistor type plugs (you probably do). Make sure your spark plug wires are resistor style as well ( they probably are). If either of those aren't the problem you will want to try running a new ground for your stereo. Preferably to the frame or engine block. If that doesn't do it your local stereo shop should be able to sell you a "noise filter". This is not the best option though because you will experience some signal loss due to the filter.
Good luck,
Judd