The rear of your car should be equipped with stiffeners that are integrated with the chassis... usually, the bumper rails are also located here. Hitches get hooked to those, not the subframe. A hitch should never be placed anywhere that will fatigue like that, it's purely unsafe.
For towing >500 lbs, you could actually drill your bumper rail and install a pintle lock-type hitch. I've done more with less...
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