If they put in an aftermarket unit, hope that they didn't cut the harness.
If they did not, and the wiring is still in tact, then it i. literally a 10 minute job to install the radio.
You may need a radio removal tool. It bacically pushes in little tabs that are on the side of most radios; that hold them in place. Its a little thin tool that slides in the side of the radio.
There is also a specific / similar tool for the OEM radio. Since you are only installing the OEM radio, you dont really need this. But you won't be able to remove it again.