I have not worked on a Bosch Impactor, but I have seen a year of weekends worth of DeWalts, PC and other drills. Normally, for what you are describing, there is a separate switch for reversing, which flips the motor polarity. There will also be a switch for the trigger, to make the motor turn.
Before disassembling, I would get the parts diagram for the parts diagram from the Bosch website or a place like ereplacementparts.com. Actually, that site has a forum, where there are often contributors who are very knowledgeable about specific power tools.
Another advantage of getting the parts breakdown, is that you can identify the cost of the suspected part before you go hacking into the case. For some controls, the cost of the control or switch can exceed the cost of a second unit. [Such was the case with a Mikita Tek Screw Gun I had, and as a result I simply bought a similar gun which was priced less than the parts I needed. Through some quirk, I now have two of them working, but it comes in real handy when helping with a deck build or similar project.]
In summary, get a parts breakdown, as it will give you information on the parts, and sometimes on the dis-assembly, before you dig in. Secondly, check out a tool repair forum, as some of the contributors are tech who fix these tools every day, and are very knowledgeable.
