I have static in the radio when I turn on the rear defroster.

The rear screen heater is a pure resistance being fed with direct current and normally will cause no radio interference as it does not vibrate or oscillate, though a loose or dirty connection or dirty switch could produce interference.

Some cars use the rear screen as a radio aerial and these are prone to producing radio interference, especially when trying to receive vertically polarised signals.
I tried this myself using a bi-fi rear screen aerial adaptor and was very disappointed with the results, the signal-to-noise ratio was appalling.

If your car uses the rear screen as an aerial there is likely to be a box of electrickery physically somewhere near the rear screen where the aerial cable originates. This box contains stuff to decouple the radio signals from the HRW circuit and will probably contain a signal amplifier.

The static you hear might just be a drastic reduction in signal strength rather than actual generated interference. You can do little else but to check the connections to and from the unit particularly the ground connections and check the voltages present on the related wiring. If no external faults are found it is likely the box of electrickery has failed.

Spending a while looking at dedicated forums might help further.