The design was never really intended for very cold winters like we get in the US. So the heater only has a fresh-air intake mounted high on the fenders, and doesn't have a recirculate ability. There also isn't much flow through the heater ducts. The heater core is too small, the truck is drafty and has a big interior to heat. These all consipire against me!
So, the only real option is to add additional heating. But I think I am going to suffer through this winter and install it in the summer. Too cold to run heater hoses and wiring under the vehicle right now.