The bracket that is already in the car works just fine nothing wrong with it, also they should have the wiring harness for it if not you can get them on ebay or crutchfield. Here are the links to them
Scosche NN04B 2007 - Up Nissan Radio Wiring Harness - eBay (item 330221221661 end time Jul-12-09 17:37:19 PDT)
Scosche NAA2B 2007 - Up Nissan Versa Antenna Adapter - eBay (item 230119254644 end time Aug-05-09 20:10:18 PDT)
And to be honest you could probably do the install yourself.