Install heater core?

Check to see if the guages are plugged in as you had to remove the dash to install the heater core. And check to see if you have any broken ground wires.

I think I just answered the other question. Here are the directions I was able to find for you. It isn't a fun job.

Disconnect the negative battery cable and properly drain the engine cooling system to a level below the heater core.
Disconnect the heater hoses from the core tubes near the cowl.
The instrument panel must be removed or repositioned for access to the heater core case. Unbolt and either tilt the instrument panel forward or disengage the instrument panel components and remove the panel from the vehicle. In most cases, tilting the panel forward should be sufficient for the necessary access, but if necessary remove the panel refer to Body & Trim in this guide.
Unfasten the heater core rear case retaining screws, then remove the rear case for access to the heater core.
Loosen the retainers and remove the heater core retaining straps.
Remove the heater core and seals.

To replace it:

Install the heater core and seals, taking care not to damage the core tubes when inserting them through the cowl.
Install and secure the core retaining straps.
Install the rear case and secure using the screws.
Reposition and secure the instrument panel and components. Tighten the cowl screws to 17 inch lbs. (1.9 Nm) and the lower instrument panel screws to 66 inch lbs. (7.5 Nm).
Connect the heater hoses to the core tubes.
Connect the negative battery cable, then properly refill the engine cooling system.
Run the engine at normal operating temperature, then check for leaks.

Let me know if this helps.

Joe

Disconnect negative battery cable. Drain engine coolant. Disconnect heater hoses at core, and plug core openings
Remove instrument panel..
Remove rear case screws and rear case. Remove straps. Remove heater core. Remove seals. To install, reverse removal procedure. Fill cooling system. Start engine and check for leaks.