Heater cores do not usually go bad except to leak or get plugged. If you
do not have fluid all over the passenger sided floor, it's probably not
leaking. And I would not really expect a 2000 model to have that much
crud in the system yet.

If you are having problems getting heat, here is the usual list of culprits:
-fan switch not putting out all 3 speeds
-blown resister pack so it does not put out all speeds.
-bad fan (bad bearings, loose ground wire)
-stuck vent doors in the heater box. This can be from warpage of the
case, bad vac switch in the heater control, missing or broken vac lines,
etc.
-bad engine thermostat (Top of the list!!)
-plugged up cooling system (flush with special cleaner, drain, refill
with new antifreeze)
-plugged heater core (backflush the heater core when you do the full
system flush)

Changing out the heater core is a RPITA. You have to remove the entire
heater box from up under the dash. Figure several hours if it your first
time through that hassle.

How about some symtoms that led you to the question??

when it goes bad it leaks. It takes some time to change out.
KH

Wet feet, puddle in the floor, windshield fogs over badly when
defroster turned on, antifreeze smell in cabin, etc.

It's a royal PITA to change.  Compared to the labor, the part is
almost free.

the dash has to come out the the hvac housing

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