When I first got my jeep it said there was a problem with the brake lights, yet they all worked. I just replaced them all (backup, turn, brake, and highmount brake) and that solved the problem. I think as the bulbs get older the resistence changes, which is how it senses an out light. not sure about that coolant sensor....
'98 Jeep Grand Cherokee 5.9 Limited.
Mods: K&N FIPK, Viper Alarm, Air horns, Kolak Ignition Upgrade, Edelbrock IAS, Tint to match back.
Stereo: Pioneer Premier DEH-P840MP head unit, Kicker 12" XPL sub, Infinity kappa 6 3/4 in front, infinity reference 5 1/2 in back on stock infinity amp.