heating problems

I recently purchased a used 1998 chevy blazer. The heater wasn't working, figured it was the blend door problem. Installed a new one from oreilys (pretty easy to fix) and heat still does not come on. The air is freezing cold, like the a/c is continously on. obviously the dial is turned to the heat side when i say this. I have hot tubes on my top and bottom hoses from the radiator as well as warm hoses from both intake and return from heater core. Can there still be a clog? the blend door break again? it seems to me like there's been both types of coolant used, can this cause the whole system to fail? truck has 91k and is being fully fluid swapped at the shop. hopefully this fixes the problem. anyone have this problem?

Here is an easy way to see if your heater core is clogged. Disconnect both heater hoses at the fire wall and attach a garden hose that has the end cut off to the smaller fitting. You can use a clamp if you want. Slowly turn the water valve on to the garden hose and increase the amount slowly. This will blow the crud out of the core and should give you a pretty good idea if the core is plugged.

Rick is dead on. I would have to do this to my 99 Blazer every other year. It was usually blocked.
You can buy a Prestone fitting that makes this job easier. It's a tee with a hose fitting. Be careful about how much water pressure you put on the heating core. The core may weaken from the buildup and corrode. I eventually had to replace the core. It is not an easy job.

Rick is dead on. I would have to do this to my 99 Blazer every other year. It was usually blocked.
You can buy a Prestone fitting that makes this job easier. It's a tee with a hose fitting. Be careful about how much water pressure you put on the heating core. The core may weaken from the buildup and corrode. I eventually had to replace the core. It is not an easy job.

That is call the Flush and Fill kit. And they work great. However, I don't recommend that you leave the Tee installed year round, as the plastic cap will crack and leak eventually. What I do is cut a short piece of heater hose and leave it on the Tee, then pull the heater hose from the core fitting and attach it to the other side of the Tee, connect the short side to the core fitting. After flushing I remove the Tee and reconnect the original heater hose to the core fitting.