the harness that is unplugged is probably a wheel speed sensor, which would cause your ABS and trac lights. those will throw a code that can be read with something like a TechII at your local GM dealer, so that will point you in a better direction. if your low coolant light is on, check around the lower intake for leaks. sometimes just an air bubble around the level sensor in the radiator will throw that light. i believe there's a procedure detailed in your owners manual about bleeding the cooling system. long story short, overfill your overflow bottle, make sure the car is hot enough to open the thermostat, and open the bleed screw on the thermostat housing until you have a steady stream of coolant. there are also two oxygen sensors, so make sure you get whether the code was for bank 1 (pre-cat) or bank 2 (post cat).