What now

No heat from heater core could mean you have a serious air pocket preventing good coolant flow in system. You have to bleed the air out, or 'burp' it. You can google how to do that.

I've seen several times now where mysterious overheating and A/C and heat malfunctioning was caused by a blown head gasket. I guess I should say "leaking" but eventually it turns into a blown head gasket. Not specific to this minivan but on several other vehicles. I hope before you actually replaced the radiator you looked into the water pump as well. If you can locate the inlet and outlet hoses for the heater core in the fire wall you can try flushing the heater core your self with a garden hose or go ahead and take it to somewhere like Midas where a coolant flush is relatively inexpensive. I wouldn't recommend them for anything else really, but they do a pretty cheap coolant flush. (Make sure engine is cold before removing any coolant lines yourself of course.)