Got a Reply from German National Tourist Office
Dear Kev,
Vintage cars are exempt from the low emission badge but the vehicle
needs to be in good and safe condition and any major alterations to the
car are not permitted. The German Automobile Club (ADAC) recommends
obtaining a 'Historic Vehicle' pass from the FIVA:
http://www.fiva.org/EN/index2.html
and putting a copy of the registration papers which show the age of the
car being over 30 years visible when parking the car in the
environmental zones. (German vintage cars can be easily identified by
the number plate which is either a black H or is a red 07 number
plate.)
Kind regards,
Ulrike Wessollek
Information & Administration Officer
German National Tourist Office
PO Box 2695
London W1A 3TN