OK, The heater you have works OK in places like California & Arizona, but may not be up to the game in Chicago, You need the under hood Heavy Duty unit.
What I was talking about for flow is Air flow through the heater core, make sure it not pluged off with dirt & lint. I don't thing you can get the blower fan on backwards.
Do you have a volt meter, if so you can chek the power input of the heater on high and see if it gets 12 volts. If all this checks out, then it's time to check water flow through the core.
