[stomachmonkey]

Before you tear into the dash you should eliminate other/easier possibilities.

1st step is to run the vehicle, get it warm.

Locate the heater lines where they run thru the firewall.

Feel them, inlet should be warm. If out is warm then think blend door issue.

If out is noticeably cooler think heater core.

If heater core disconnect lines, back flush then forward flush then back flush again.

Another possibility is if the vehicle was low on coolant you can have air in the system. You need to burp it.

Park on incline or jack up the front, run it with the coolant reservoir cap off. Look for air bubbles.

Another common one, if the vehicle has digital climate control the control module could be fried.