In most cars here in PK, mechanics disconnect the heater core saying it will effect the cooling of car. so when they disconnect they dont flush / vaccum it which results in rusting of the heater core.
i would suggest, go to an ac wala, get the whole system fixed including the ac and heating system. because for replacing heater core, he will open the complete system beneath the dashboard and so he will charge the labour.
he will also arrange for you heater core for alto vxr.
if ur ac is working then ur kharcha will be minimum and do ask the ac wala about the whole process.