Thanks Jim, this is what had baffled me, as Germany had several touring car championships all happening in a similar time-frame in the early 1980s, including DRM, DRP, DRT. The DTM (Deutsche Tourenwagen Meisterschaft) didn't begin until 1986, but it was essentially just a renamed version of the Deutsche Produktionswagen Meisterschaft), which itself only ran for two years, 1984/85 and had grown from the DRM.
I have only been able to find one Group A Mustang that raced in the DPM in 1984, run by the ABR Ringhausen Rennsport team, driven by Manfred Trint. This car was quite competitive in 1984, but the pictures I have seen of it, the car doesn't look the same as the Johnson car. I also thought the Zakspeed car would have been used in the DPM, but can't find any reference to this, which is why I wondered where Johnsons cars had raced.
However, Zakspeed did race a Mustang at the touring car support race at the 1983 Hockenheim GP event, driven by Klaus Niedzwiedz. Could this possibly be one of the cars bought by Johnson?
You can see a photo of the Trint car here: http://touringcarracing.net/Pages/p%...alenpokal.html