2006 sprinter 2500 no heat in van blows cold air

What is your question?

the van needed glow plugs and we were putting off but the van stopped heating and we brought in and had the glow plugs replaced. Dealership said could be 1 of 3 things but couldn't tell us which it was. the van has 190000 milesi want to fix but dealer want to trhow almost 4500.00 at it just for parts.looking for a place to start trouble shooting.