Thought that might be the case, since the recess where the radio was fitted did not look ‘chopped’ about, as it might do after someone’s attempts to enlarge it.
The heater is mounted at an angle behind the dash. Thus there is more clearance at the top, but none (well, only enough for the switches) at the bottom.
If you wanted to modify the dash to fit a double din unit, for instance to install a navigation system or other such double high application, then you would have problems.