2006 sprinter 2500 no heat in van blows cold air on 2006 Dodge Sprinter 2500

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.

Asked by for the 2006 Dodge Sprinter 2500