3D and 2D rendering

Imagine you have to simulate a vehicle with a GPS panel. How do you simulate the buttons on the GPS?

I have a vehicle that moves, rendered from a main camera, then a second camera, this one orthographic and moving 1,000 meter over the vehicle, rendered on a target texture (yes, I have Unity Pro) and that works well. When I click, from the main camera, on the GPS screen, the cameras are switched and I the camera texture is replaced by null so that the entire GPS covers the main screen, until a "Return" button is pressed.

The problem is that, now, I have to display data on that GPS screen (speed, heading, position, etc.) and also add some interactive buttons to toggle e.g. head-up to north-up display.

How can I do that? Using OnGUI doesn't work because it goes on the top of all cameras. Thanks in advance.