Alan,  I have three suggestions for you.

First, find someone who is a native German speaker (assuming that
the text is in German).  Often times, a person who reads in the same
language can recognize words with missing letters or other details.
Whereas a non-native speaker must identify each individual letter,
a native speaker and reader can use surrounding text and other patterns
to do a much better job.

Second, sometimes light of different wavelengths can bring out writing
that typical sunlight or room light cannot.  The easiest thing to try
is the different types of light you already have in your home.

Ordinary incandescent bulbs are heavy in the red/yellow end of the
spectrum.  Cool white, warm white, gro-light, compact florescents are
each a slightly different color temperature.  Novelty lights like the
UV ones kids use to see posters are another possibility.  You can just
buy a small bulb for an existing lamp.  Better yet, get thee to a rock
shop and try one of the better UV lights which have both UVa and UVb
spectrums.  Perhaps one of those will bring out the writing.

Another approach is to use a viewing device that is differently sensitive
than your eyes.  Different color filters will also help in some cases.
A simple way is to use samples of colored plastic or camera lenses or
theatrical light filters.  Or try a video camera that is IR sensitive.
Some of the popular brands were IR sensitive a few years ago.

Lastly, maybe you can find someone in your local museum or police
department who has access to the right light analysis equipment.  They
will often help if you ask nicely.  A friend of mine took a toy player
trumpet to work for me and X-rayed it at the hospital where she worked.
I hope you find a solution.  This is not a rare problem and we all
might benefit from your solution.

Regards from snowy upstate New York,
Craig Smith