According to Crutchfield it seems you lose the navi if you replace the headunit. If nothing else, I'd assume a control button is probably part of the dash you wind up replacing with the dash kit to install an aftermarket.
A few weeks ago I picked up a 2009 Touring 5. I found the Ipod connectivity among other things fairly archaic(lol2) so I decided to install the Pioneer AVH-4400BT from my previous vehicle in the 5. Had to pic up a new steering wheel control input adapter, the standard metra faceplate and...
Definately confirmed that you need to either steal the two steering wheel pins off a second connector shell or steel 1 + move 1 unused pin (amp turn on not used). Last weeked I did 95% of the install of a pioneer AVH-4400BH along with the XM receiverback by the spare tire and a U220 nav add on...
$46 @ Amazon, Crutchfield tends to be pretty high on everything despite the "free" included items. On the other hand, they ship very fast and provide generally good installation notes, which other do not, and a decent tech support department when questions arise half way though <g>.
There are 2 popular aftermarket control boxes to mate aftermarket head units to the steering wheel controls. One is called PAC and the other is the ASWC which is the one I have used. The ASWC generally programs itself so it is a little easier to install.
Just bought a 5 over the weekend, haven't torn in to the stereo yet but I will in a week or so. I'm fairly certain this is going to be the same as my MX-5 meaning you would need 2 connectors in order to harvest a couple pins off the extra shell and install them in the firt to get the steering...