Full translations please, 1st on the back window, 2nd on the rear door, and finally on the number plate.
The "Münchgesang" is a business/ owner's? name.
As many German names, it's a word/name made of 2 parts, each of which a word with its own meaning.
Direct translation might be "Münch-Song" where a Münch was the first ever "Gozilla"-Bike, in the vein of a Hayabusa.
Bike of the '70's, handbuilt, today only exists in private collections and museums.
Now priceless due to its rarity (which I helped, crashing one + bending it to the shitter)
en.wikipedia.org
Then again, "a Münch" was also (historically) a monk.....like Munich/ german München.... got its name from the local "monastery".
And monks can and do sing....at least they did in the old days.
The sign on the rear door is a "Wanted" sign for a ....nowadays... fitter+turner?
Number plate? Doesn't mean anything to me. BI is for a City of Bielefeld registration.