Okay so paid options first.
Here's the Kenwood to ISO adapter for that unit ($16 delivered):
http://www.ebay.com.au/itm/ISO-WIRING-HARNESS-for-KENWOOD-KMM-BT302-KMMBT302-adaptor-cable-connector-lead-/272308937683Note that it specifically references the blue and yellow steering wheel control wire - which is also in the head unit manual on the wiring/installation diagram page.
Now here is the vehicle to ISO adapter for the 2010 GLXR ($15 delivered):
http://www.ebay.com.au/itm/MITSUBISHI-TRITON-14-PIN-ISO-WIRING-HARNESS-adaptor-cable-connector-lead-loom-/251887138744You should confirm first that your car has the 14 pin plug. Note that the description specifically says that this loom will not enable the steering wheel controls.
The steering wheel controller interface that the same seller does for the Triton is this one ($115 delivered - a lot of money but way less than the $200 you mentioned earlier):
http://www.ebay.com.au/itm/MITSUBISHI-TRITON-ML-MN-2006-2011-STEERING-WHEEL-CONTROL-HARNESS-PATCH-LEAD-/271579910600Note that they list manufacturers it will work with and Kenwood is one of those. They suggest you tell them which model you have as there are variations in the patch looms to match the brand. The picture shows an example with a small plug like a headphone plug which wouldn't fit your unit. I'm guessing if you tell them the unit you will get one with a single output wire to match the yellow/blue wire mentioned above.
I think, but you'll need to check, that the steering wheel control loom probably negates the need for the vehicle specific loom I mentioned above. It looks to replicate the ISO plugs and the plug into the back of the head unit. If that's right then if you take off the $15 you'd pay for that loom anyway the steering option is really only costing you $100 extra.
Now the DIY options.
This page from Kenwood gives some insight to how simple the whole interface is
http://www.kenwood.com/au/car/visual_navigation/oem/ It doesn't actually help you though as it only has the 20 pin for Mitsubishi and not your 14 pin plug. But - from memory that head unit has two plugs going into it. I've got one somewhere here I can look at but I can't do it all for you
and I'd have to walk 50 odd metres and then rummage around a bit so that's too hard for me this early in the day. So have a look at the other plug and see if it has 20 pins. If it does you might be lucky. It may be as simple as connecting the wire identified to the blue/yellow wire.
The ebay seller seems to contend that their loom has a magic black box which translates the signal to one that matches the head unit. I'm not 100% convinced by that. It probably varies a little from head unit to head unit but you're probably in with a shot of a direct connection actually working without the magic black box.
This search will lead you to 10 threads specifically about steering wheel controls - if the answer as to which wire and plug you need isn't in one of them I'll be stunned.
http://www.newtriton.net/phpbb/search.php?keywords=steering+wheel+control*&terms=all&author=&sc=1&sf=titleonly&sr=posts&sk=t&sd=d&st=0&ch=300&t=0&submit=SearchYou also mention the temperature control thing and the bluetooth. I'm guessing you mean the RV Meter (the trip computer thing above the stereo) with the temperature reference. That will continue to work with the new head unit. All you will lose, if you had it, is the display of material that related to the radio or CD player. So station and track number, that sort of thing. No great loss.
The bluetooth I'm not sure what you mean. The new unit has its own bluetooth built in. And its own microphone that plugs into the back. Effectively you're doing a total transplant there replacing the existing setup with that one.