I put a lower radiator heater (the Diesel Giant kit) in my '85 and it works well. There's enough flow and heat transfer through the thermostat to warm the block. It's not as efficient – the radiator heater has a 600 Watt element while the factory element is 400 Watts but it does work.

I have also heard that the factory plug is a bear to remove. Warming the block with a propane torch might help. My California version with the trap cat means you can't even see the plug much less get at it.

You might consider installing a radiator hose kit and selling the factory heater on eBay – that's what I did a couple of years ago.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970