Prefs.dlg and RearView.cs go into the Starsiege directory.
open up AutoExec.cs which is located in the scripts
folder. Add the line

exec(RearView);

When ingame press F11 to bring up the preferences. Adjust the camera window
position, width and height to what you want then press "Save". You can also adjust the
camera fov by moving the slider.

If you want to be able to toggle the rearview on and off. Do not put
exec("RearView.cs"); in AutoExec.cs. Instead modify your keymap.

bindCommand(keyboard0, make, control, "r", TO, "Exec(RearView);ToggleRearView();");