Tacoma loses heat below -10°C; thermostat changed, fan clutch issue

Once you verify that the thermostat is not the issue take one of the hoses off the heater core connections and start her, if coolant comes out the hose put it back on the heater core and take the other hose off and start her again, you are looking for coolant flow through the heater core. Takes maybe 3 minutes and costs nothing.

 

Except introducing air to your system. An easier way would be to feel both hoses when the heater is running. Both should be hot if there's flow.

 

In rereading OPs original post, I believe I have misunderstood part of the issue. OP is getting heat to the cab and is just complaining about the truck not getting to temp in the winter. So the heater core isn't going to be an issue.

I still stand by incorrect or incorrectly installed thermostat. Not really sure about the fan clutch idea as usually that causes overheating issues when those go bad. Not to mention usually the thermostat would be able to regulate that to a degree.

 

When diagnosing, check everything.

 

Makes me glad to live in Florida. As long as the a/c works then 10 months out of the year are comfy