A schematic or wiring diagram will not help your cause. You would need the complete service manual for that radio if it exists. That manual would spell out how if indeed the removal of the cassett mechanisim is really difficult to get the bulb. In electronic fabrication the germans had some funny ideals on occasion.

You could get the same radio out of a junker at a yard and practice or at least see if it is feasible for you to do it yourself. Other than the two above mentioned items I would farm out the hard bulb change. If there is a becker forum in english unless you speak german might be worth a try as previously mentioned.

I stopped repairs on car radios long ago when I found it took far too long to get into the problem area. Before that era emerged access to the problem area was usually only a couple of minutes or so. It just became too time consuming. Stacked electronic boards and hard to get replacement parts where the last straw for me.