just a couple of ideas (this is in general, not specific to DII as i haven't had it long enough)..
Start from the beginning..
does the heater Core heat up?
eg.. run the car and turn on your heater and when the engine is warm, reach under and feel the heater core (be careful, its very hot)..
or you can feel the coolant lines feeding into the firewall under the hood,
are they hot (both the delivery and return hose?)
if they are cold, then there maybe a small device that diverts coolant to the heater core only when requested, if this is broke it will never divert the coolant and the heatercore will never get hot. (older cars don't have this)
if they are hot, touch te heater core (very hot) and double check the core is indeed heating up. then i would then move onto check the heater fan duct.. the flap that directs air from HOT or COLD, if it is sticking or not actuating (common problem in other cars is a vacuum leak on the actuator) then you need to look there..
if your heater core is getting hot, and the fan duct is working properly and you still aren't getting heat then i would be stumped, but if you have the above then its something else entirely..
