97 kia sephia,heat and air

the air does not blow out that cold,and the heat doesnt get that hot,we did put new heater core on it.it is so bad that we cant even defrost the windshield,and the defrosters dont work in the back.has anyone came across this before?i really need to get this thing fixed as soon as possible before it gets any colder,lol.so any suggestion would be very helpfull.thanks

cold air problem: a major refrigerant leak or a/c wiring problem.

Open the hood of this vehicle and look for something fairly
obvious like a ruptured A/C hose or a broken compressor
belt (refer to a manual covering this vehicle if you don't know
where the compressor is). If you can't find anything, try
checking the circuit breaker (refer to this vehicle's owner's
manual for instructions pertaining to this). If it has popped,
reset it. Hopefully it won't pop again but... realistically, it
probably will. Why? Because, whatever caused it to pop
in the first place hasn't been fixed... right? So, it's likely it'll
happen again. Therefore, take this vehicle to an A/C
specialist and have him examine this vehicle's electrical
system.

heater problem: heater control valve isn opening

Take this vehicle to a shop specializing in heater repair and
have the mechanic inspect both the heater control valve and
the device that activates the valve. Upon his diagnosis, have
the necessary repairs done. In this particular situation, some
mechanics may suggest replacing the heater core in addition
to fixing the heater control valve problem. They'll claim the
heater core is clogged and therefore is contributing to a low
air temperature heater output. Although there may be some
truth to this, nine times out of ten, fixing whatever is wrong
with the heater control valve will dramatically improve heater
performance. In other words, in most cases, replacing the
heater core won't add that much to the heater's performance.

hope this helps

Terrell