The cockpit.obj file for steam gauges is called Cessna_172SP_cockpit.obj while the glass is Cessna_172SP_G1000_cockpit.obj. They are both in the root directory of the airplane.
Under objects in the root folder of the airplane, you will see objects. Under Objects you will find the instrument folder. There are the 430 and 530 objects you need to edit.
On plane maker, make sure you open the right version of the 172 you want to edit. Steam or Glass. Under Standard, select Panel 3D. Select G530 on the top right side under hierarchy and on the bottom bar you should see the CoPilot CheckBox. 530 should be checked, 430 unchecked.
Hope this helps