98 blazer heat works sometimes, i think it is in the hvac controls. I can start the truck heat will work then slowly go to cold. I can turn the truck off restart it and heat works for a minute and does the same thing. I just put in a new radio and it did it before that. But while i had the dash apart and the truck running for heat because it is cold out i could unplug the controls and plug them back in and it would work again with out shutting the truck off. Any insight is appreciated

 

Sounds like it could be your thermostat.
Could be things are not circulating properly.
There are a couple large hoses up front connected
to the top and the bottom of your radiator. When you have heat
turn the car off and touch both hoses. Both should be warm if the
coolant is circulating properly. When you lose heat, let the vehicle
run a little more and check the hoses again. The correct running temperature
is around 195 degrees. If you thermostat is closing up and there is no circulation
one of the hoses will be very cool to the touch. This is a start.

Mark K

 

check all your vacuumlines sounds like yu have a leak

 

^^^^ i second that def sounds like a vac issue.

 

or you could just leave your wife home

 

Let's look at evidence and clarify:
OP says at start heater blow hot, then gradually cooler till cold.
If engine stopped and restarted, does same again.
If control harness disconnected then reconnected, does same.

Judging from that.. I assume
It is automatic climate option rather than manual set heater.
Stop-start sequence described is near immediate
So..
- It is not a clogged core. That would go warm to cold in a second or two and would not get warm again in just a second or two.
- It is not low coolant or there would be air bubble gurgling, and would stay cool till engine got well over 100 deg.

What if the orig setting was '70 deg' and it was warm then gradually cooled, then if setting was changed to '85' it would warm then cool as above

What that would tell me is that there's a problem either with the Blend door and its actuator or with the temp sensors either cabin or plenum.

Since there's plenty of posts on fixing blend door and replacing actuator, that's where I would bet.
Here is a pretty good basket of common fixes that hides in the build forums
Scroll down for one of best actuator fix posts I've seen, including youtube videos on it.
https://blazerforum.com/forum/builds...t-38396/page2/