Re: 2007 grand mi gig?
you will need the low speed unit, and a lockpick mygig upgrade adapter harness. If you currently have the REC you will need to get a bezel filler, if you have the REQ you will have to do some dremeling to open up the hole in the dash. If you want sirius you will need to run an extension cable all the way to the antenna in the rear because the mygig uses a different connector. You will also need to get a gps antenna, which are reasonably cheap.
As far as acquiring an RER, your best bet is ebay. Ebay is also the best place to get most of this accessories, especially the lockpick adapter.
I am actually just finishing up my RER upgrade on my 2007 from a REQ, including installation of a backup camera, so I speak from experience