I actually wrote that card myself one or two years ago and now I'm trying to decode it.
Starting time: 10:13
Well, it wasn't that hard:
"Moltke bläst in's Waldhorn" is the main hint.
To understand it, you have to live in Karlsruhe, where there are both a Moltke- and a Waldhornstraße (street). For further help, the drawing on the lower border shows Moltke- and Waldhornstraße how the ends which end in Schloßplatz/Hans-Thoma-Straße would appear on a map. The code itself are the streets in between, which are all running in the direction of the Karlsruher Palais:
Bismarck-, Akademie-, Wald-, Herren-, Ritter-, Lamm-, Karl-Friedrich-, Kreuz-, Adler- and Kronenstraße.
The next hint is the sign for average, ⌀.
There is only way to average letters, which is to assign a value to each letter.
Here, I chose the most simple way, every letter has the value of it's position in the alphabet, a is 1 and z is 26, so you have to translate the street names an the text on the card. If there are two letters instead of one, build the average of them. This may be discovered because every time there are two letters, they are adjacent (safe for one typo).
Every word on the cart has to be added to one streets (from east to west), an example for the first word:
He is H(8)-E(5)
The first two letters of the first street (Bismarckstraße) are B(2) and I(9).
If 8=(2+x)/2 then x=14 and
If 5=(9+x)/2 then x=1
In the 14th position of the alphabet you'll find "N", in the first "A", so you got the first word "Na".
Just do it for every word and tadaa, you know what I wrote a long time ago.
The whole thing was unintentionally made harder due to the fact that my "L" and "C" are hard to tell apart and that I made a typo and two errors.