Webasto Thermotop was I believe factory fit, not cheap though, various models, some just need a 12v signal to start, and others need a canbus or wbus signal to start.
You can get cheap chinese air/coolant heater for a lot less than the price of a Webasto unit.
Webasto and Eberspacher cost a bloody fortune in spares when they go wrong, and the Thermotop does like to go wrong, have had two ecus fail over the years, and my D3 one recently died for the same reason (making it 3 failures) though a 2nd hand one was less than a 100 quid.