I am on the shop right now with same issue on an 04. I was quoted about $420 from an independent that I trust...but I feel he is a little high. It's a pump, 3 hrs of labor and 2 g of antifreeze.
Engine diagrams can be used as guides.. An engine diagram to a 2003 Chevy Trailblazer can be found in its maintenance and repair manuals.
It could be coming from your heater core, may need to repair or replace it.
This answer will vary with what's wrong and which shop looks at it.
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buy a haynes repair book at your local parts store it has detailed instrutions
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No