I have a Webasto heater (Diesel) in my 300CD which I transplanted from my '77 300D. I have used it for a few winters here in Canada. At -30C it will bring it from cold to almost operating temperature in about 30 minutes.

It's a very efficient little machine, easy to install. The only drawback is the cost around $1200-$1500 new. I bought this one lightly used for $500. It's never required any maintenance and has never let me down. Definitely worth it if you have a spare chunk of cash.

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1984 300CD 155k miles
1983 300TD 199k miles
2005 Legacy GT Wagon