Radiator fans won't turn on unless ac is running. why does it do that, how can i fix it?

There are 2-3 sensors that will trigger your electric fan comming on, 1 is usually near thermostats, 1 usually near bottom or on side of radiator, third is on your ac compressor. With car running turn on ac fan should kick on if it does ac sensor is good. Then it will be one of your other 2 sensors

Check the sensor in the bottom of the radiator is connected and working.The water has to get hot before they come on near boiling point,the air con switch over ride s the normal sensor.

There is an electric fan that should come on with the AC and run whenever the AC is on,if it doesn't come on it will heat up very quickly. Sounds like a problem with that radiator mounted fan.
The fan is supposed to come on when the engine gets hot enough also.
If so, the AC should be ok at hiway speeds.But you need the fan to work.

Start the car and turn on the AC to max. Do either of the fans run with the AC compressor engaged? Also, there is a temp sensor for the fan circuit but I don't know exactly where it is located on that vehicle.

You have to thank the Gauge that shuts down your engine when hot bec. it wont knock...you have to clean your radiator and see if the coolant inside it run through the radiator holes to the engine water jacket to make sure your engine will not over heat...For the fans, find the fan relay...it's under the hood in the fuse box...

The fans are radiator cooling fans. They are temperature controled so when you first start your engine in the morning they should not run - UNLESS you start it with your AC on. If you turn your AC on one of them should start operating and the other one will kick on when your engine starts getting hot.

if you turn on the ac the other fan should come on, However I don't believe that will cure the problem your having since having the ac fan and the ac turned on pretty much cancel each other out, Putting a new thermostat on Just allows the thermostat to open and close at the set temp that's on the thermostat , Not really fixing the problem, ( unless the thermostat is stuck in the closed position would replacement of it help ) what I would suggest is to completely flush your cooling system, and add new antifreeze, antifreeze like oil breaks down, other heating problems can be caused from the car not tuned properly, check the timing , and components associated with the car running properly, ( plugs, rotor and cap, spark plug wires, )

You may have one conventional belt-driven fan with a thermal clutch and the other electric or, both could be electric on your car.
The cooling fan that operates during non-AC operation (just a running engine) if electric, will be controlled by a thermal switch in the lower radiator tank and may not turn on at all unless the engine is warmed up. The belt-driven fan should be turning at some speed all the time if the engine is running and at much higher speed with a hot engine.
The fan that cools mostly when the AC is operating won't run under normal operation but may turn on in hot weather (or when the engine is under heavier loads) even without the AC operating.
Checking them would require having a warmed-up engine and the AC turned on. Both fans regardless of type should be turning then.

try to see the connection if still good..replace it if not...and see the fuse if still working

The fan relay may be malfunctioning (the fan relay is what actually switches the fan on and off and gets its signal from the ECM, which in turn gets its signal from the temp sensor) testing/replacing the relay and temp sensor would be the best thing to start with,... unless you notice any additional (non factory) wire attached to the power wires to the fan.  Sometimes folks, in an attmpt to fix  an overheating or fan relay problem, attach a ignition switched positive lead from the fusebox or other switched source to the positive poer lead on the fan,.. making the fan run whenever the ignition is switched on.