Hard wiring into the jeep's electronics is not advisable. A harness costs no more than $20 and requires no more (and probably less) work than hard wiring. You won't have to figure out any wiring and doing all the wiring on the short leads that Chrysler gives you will not be fun. You can wire a harness carefully and easily on the bench in your garage. It will also be a royal pain if you want to put a different radio in later.
Well worth the investment IMO.
Edit: You'll need to purchase a dash kit to make the radio fit anyway. What's another few dollars to do the job right?
by the thing that spins when you drive underneath