If you haven't done so already, check out the nav installation instructions available here. If your Crossfire is a 2005 or above (as your sig says), you shouldn't need to tap the speed sensor or backup signal as shown on pages 5 and 6 of the installation instructions. Your car should already be wired. To be sure, look at the wiring harness that goes to the back of your head unit and compare it to the diagram in step 12 on page 5.

When you say the fellow is going to include the antenna, I presume you mean the GPS antenna. That's good! Many people don't get that because most sellers are unwilling to disassemble the dash to get to the antenna. Otherwise, you'd have to order one from Becker or Chrysler.

The nav unit has a different connector for the AM/FM radio antenna than the non-nav unit. If you're doing a swap, the recipient of your non-nav unit won't need the adapter (it won't fit on the non-nav unit) so be sure he sends it to you. This is part E on page 1 of the installation instructions. If he doesn't have it or won't send it to you, you can't order it from Becker, either. They don't stock it. You can make one by combining two other adapters. You can order an adapter that converts the antenna jack on the back of the nav head unit to a Motorola female from Becker. It's part #1215.663-881 and costs $3.79+S/H. Then, you can order a Euro female to Motorola male adapter from Crutchfield. Combine the two and you effectively have part E.

I hope that helps.