The important thing to know about the German scene is that every state in Germany (Berlin is one of them) has their own idea about commercial archaeology.
Each 'Landesdenkmalamt' (State Archaeology Service) has rules about whether private companies can tender for excavations and surveys or not. This is because there are differences in legislation from one state to another. Some have a PPG16 - like rule, but others don't.
That means that in some states, including Berlin I think, commercial archaeology is possible, in others the State Archaeology Service or Universities take care of all developer related field archaeology.
Generally speaking, commercial archaeology in Germany is still very much in its infancy, with many units displaying appauling standards both in terms of employment regulations, pay and field standards. Most companies are very small affairs, i.e. they consist of one or two full-time employees who own and run the whole thing. Everyone else is on project related contracts, although some larger units have begun to establish themselves in the past ten years. There is no BAJR type website at the moment, but you can check
http://www.archaeologie-online.de and the forums there for occassional job searches (it's all in German I'm afraid). There is also
http://archaeologieforum.at/jobnew.php, which is a proper job site, but only few adverts relate field jobs.
Working for State Archaeology Service can be much more enjoyable, because employment is often more long-term, but it is VERY difficult to get into and very bureaucratic. Even as a German getting a job in a Landesdenkmalamt it is mostly only possible via very good relations.
Saying all that archaeology in Berlin has in recent years been dominated by WWII archaeology. Many old air raid shelters have been investigated in the run up to masive construction projects in the city center. There have also been a number of early medieval (slavic) settlements in the city limits, as well as some prehistoic archaeology.
There is a lot of work going on in Brandenburg - which has a commercial sector - on road projects and the like, and it is basically just outside Berlin.
The good news is that most good archaeologists in Germany have a high opinion of British field archies. When I used to work on sites in Germany British field archaeologists were often regarded as the cutting edge, and given what I have seen in terms of standards in German field archaeology in recent years this probably still is so.
Perhaps it is easiest to write to a few companies directly.
You can find a register of the most well-known companies in Germany and their addresses here:
http://www.uni-bamberg.de/index.php?id=8806
Good luck!