Negative ion generator scrambles with other electronic components in device, how can I put it in properly?
Here are some suggestions.

All metal parts in the air purifier module are to be earthed. This includes the metal housing of the ionizer circuit board.

In case of need to work on the ionizer circuit, it must be powered off and the HV pins shorted to common (-ve of 12V= PSU) to discharge the unit completely and avoid unpleasant HV shocks.