On "High", the temperature is controlled by a thermistor within the grid (element) inside the seat cushion. Once the theremistor reaches a certain temperature it goes open until it cools down. After cool down it closes and completes the circuit until it heats up again. This process repeats itself.
I've seen theremistors fail in the "closed" mode meaning that no matter how hot it gets it never "opens". Semiconductors are like that sometimes.
I don't know what is in the control module but on "Low" it probably controls the temperatue by pulsing the ground circuit.
I can't see how a ground issue would cause what you're seeing. If it were a ground issue then it wouldn't heat up enough.
Can you smell the wires burning when the switch in on "Low"?
