Heat

the middle vents are designed to give only outside unheated air, but the wheels at the side of each vent should shut them off totally, maybe they are broken.if you remove the dash top you can get them out by removing the two srews you can see on them.
if you have air coming from the bottom of the console then there is probably some vent tubing misplaced, etc.
the standard heater on a 280 should be able to give out very warm air, it could be you have an airlock in your heater matrix, try disconnecting the top hose in the engine bay that goes to the heater and filllng directly with the heater set on hot.you may need to disconnect both hoses and connecting a garden hose to it to flush it and remove any silt and muck in it.do this in one hose then the other a few times to make sur you move as much out as possible.
check that when you move the lever in the centre console that it is actually moving the valve in the engine bay, possibly remove the valve and make sure it is not gunked up and operates ok even if the outside of it is moving ok.
these are all things i have done to and found wrong with g heating systems over the years!!!!
let us know how you get on.
regards

I'd consider investing in a Webasto too...

It's a programmable, stationary and auxiliary heating that uses the same fuel as your G and greatly increases comfort in extreme climates such as yours...

http://www.webasto.com/products/en/3119_3387.html

There's the Eberspächer, too:

http://www.eberspacher.com/waterheating.php?section=waterheating

Should be able to get one on eBay.

I think there is a distributor in Edmonton or there abouts - though not that convenient to Whitehorse..../