try this just for now, you should see two hoses going into the firewall in the engine bay (they'll be right next to each other, on the passenger side) pull both of them off and your gonna wanna find some way to connect them. when mine went, i went to home depot and got some copper fittings and pipe and made a u shape connector (soldered them together of course) and it by passed the heater core. if that fixes the problem then you know what it is. depending on how cold it gets where you live, yo might be able to do without it, and it actually help your ac run colder. if you need it, you could just bypass it til it starts to get colder so you could save up for a little bit, its definately not something to try your self, atleast it wasnt on my 97, i took the whole dash out, and still wasnt able to get to it fixed. and i dont think its all that cheap to get fixed.
if that doesnt fix it, which it prob will, your head gasket might need to be replaced, but try the bypass first.