I have a 92 z/28 with a 350 tpi engine. it overheats when i drive it anywhere but the highway. i have determined that neither of the electric fans will come on, even if the heat gauge is almost red-lined. I have tested the fans by hooking them up to a battery, and they both work. What else could be the problem? What would it take to fix it , and how much is it going to cost me?