Gday Peter
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High-side switching - meaning you interrupt the 12V rather than the ground return - is the way to go and can avoid some EMC issues as well.
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I have made a fair range of dew heaters now, both analog and digital control, with and without thermostatic control ( incl humidity / dew point control )
In nearly all cases, if you use PWM you will get RFI.
Even new commercial boxes marketed as having low RFI are affected
The main problem is not the box, its the wire going to the strap
as that radiates away like blazes, and PNP vs NPN wont fix it.
Secondly, with PNP switching, you need more circuitry if you want to use a microprocessor to drive it. Again a tradeoff
For anyone wanting a very simple 0-100% manual controller
an LM324, a pot, a few resistors and a mosfet is all thats required.
I have attached a piccy of the simplest one i make
Once you have the LM324 configured as a sawtooth generator
its a simple matter to add multiple outputs by just adding more
LM324s set up as comparators to drive the Mosfets.
Very cheap.
Andrew