why does my car die after the needle hits 500 RPMs and runs rough while idling

Many possible reasons for rough running.It does need to idle at 800 rpm-below that it can quit.Easiest remedy would be to replace spark plugs.Frost on inside of windshield suggests a leak in the heater core.Do you smell antifreeze when the heat is on?

I would lean against spark plugs as being the cause of your low idle, and look more at the idle air control valve. If this valve is sticking at all, it can allow the engine idle to roll or even be too low. You could have an issue with the drivers door seal. An abundance of moisture in the vehicle all the time (wet floor mats, damp weather with door seal issue, etc) will cause moisture to collect inside as well as outside the car. Make sure your interior is often dry and free of dampness.

On any taurus manufacturered before 02 you can pull the spark plug wires one at a time from the coil pack (while the car is running) to see which plug is not firing as you do this make sure to pay close attention to the coil pack. (Place each wire back on its distributor plug as you go). On taurus's there is three ignition coils for the six cylinders. The car fires on both cylinders in the pack at the same time causing the coils to be very hot. a bad coil pack will cause the vehicle to run very rough. When you pull a wire look at the pack if you see electricity arcing in between the spark plug wires the coil pack is bad. If it's not the coil pack. By the time you check all the wire one by one you'll be able to figure out which cylinders aren't firing. The firing secuence is 1-6, 2-5, 3-4, or- 1,2,3 6,5,4, if the car misfires to the point that you can notice it you can take it in and have the codes ran. (Note if the car misfires at all the diognostics center will pick it up and the check engine light will come on. If you start the car and drive it three consecutive times without any more additional problems the light will shut off but the codes will still be there) if you have for instance 1-6, 2-5, or 3-4 misfire codes it will be the coil pack, for these cylinders fire at the same time, if you have a misfire on cylinders 1 and 4 they are in separate coils on separate sides of the coil pack so they are not likely to be connected by crossover arcing, unless the silicone on the back side of the pack is bad, but the coil in the middle will be the most effected by this form of crossover.