Heater only blows cold air. I have replaced the thermostate and blown clean the heater core. also replaced a control switch ($280). Sick of getting ripped off.

When the van is warmed completely check the top radiator hose by squeezing it together. If you can squeeze it without any resistance replace the radiator cap. The coolant is not circulating properly and can keep the heater from working. If you cannot squeeze the hose together than you may have a bad water pump. The water pump has fins inside of its' case. Those fins may be rusted completely off to where the water cannot circulate. You can check the circulation by feeling the top and bottom hose coming off of the water pump to the radiator. Both those hoses should be the same temp. If not I would consider changing the water pump.