I am in NY city it has been very cold lately. I had the same problem but mine was even worst cose it would tale 45 mi before the needle moves above 40C. And only lukwarm air even when I put hearter at max.
Found out, it was stuck open thermostat. Thanks to this forum. Changed it, and now takes 5-10 min to reach 70C and about 15 min to reach 80C. Warm air starts within 3-4 minutes. I have to smooth her out by reving to 1500k after a minute of idle.

You might have a partially open thermostat. After your car reaches 60C or so touch the upper hose and feet the heat. It should be cold, if its warm you have an open thermostat.

See the posts by MVK , I have some nice photos of the stuck open t-stat.

Good luck. Keep us posted with the progress.

MVK

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1985 300D Turbo 179,000miles