For anyone thats interested I had been looking into fitting a 12v coolant pre-heater to my T5 (2005 2.5 Tdi). I had dismissed any of the fuel heaters as it just wouldn't get used, I just want to improve the warm up time and demisting.

I started looking for 12v electrical heaters. I couldn't find a VW version but found that BMW fitted a pre-heater to many of their vehicles around 2004 onwards.
They're cheap (used £10-£30) and you can get them with the coolant pipes and connector which you can modify to use. Search for 6918806 on ebay.

The heater side connects straight to the battery and the control side uses a +12v feed and PWM control signal. The PWM control signal (seems happy with frequency of 500hz-1khz) ranges from 10-90% duty cycle which gives 0 to approx 120Amps, although I haven't tested at the maximum range.
I've run the heater at around 70amps and because it's placed in the incoming feed to the heater matrix gives you demisting heat quickly. It also increases engine warm up time.

The supply for the control comes from the Aux Coolant Pump drive so will only engage when engine has been running for around 20 seconds. I have attached pictures of the install. The heater is mounted to the scuttle using some threaded rod and angled aluminium. At the moment I have a DC current meter so I can check whats happening.

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