What is the fine-grained topic of the following text: His vast experience comes from his 12 years spent at AMG well before the tuning company became part of Mercedes-Benz in 1999, then going on to serve as that AMG’s North American technical director prior to branching out on his own to start RENNtech in 1989. Only return the topic.
The person's professional experience in AMG and founding RENNtech.