The heater hot water control valve on an S-10 Blazer is located
inside the passenger compartment, typically under the dashboard. It's part of the heater core plumbing. However, it's not always easily accessible; you might need to remove some parts of the dashboard or access panels to get to it.
It's difficult to give a more precise location without knowing the specific year of your S-10 Blazer. The layout of components under the dash can vary slightly between model years. A repair manual specific to your year and model will be your best resource for pinpoint location.
