I am really bored so I did a little thing about German boutique manufacturers. This was typed on a phone so it was a pain in the ass. Sorry for any grammatical errors and typos.

The Keinath GT/C was a sports coupe by an Opel dealer turned car manufacturer. It was based on the Opel Omega B (aka Cadillac Catera) and featured its 3.2 V6 engine that powered the rear wheels.
Isdera which is short for Ingenieurbüro für Styling, Design und Racing and was found in 1982. Pictured above is the Imperator 108i which was built from 1984 to 2001 and was powered by Mercedes V8 engines such as the M117 V8. The design was heavily inspired by the Mercedes CW 311 concept car. In fact, Mercedes Benz granted permission to Isdera to build the car as Mercedes had no interest in building it themselves.
The Roding Roadster was introduced in 2008 and is ultra light with only 920kg. It features a 235hp BMW sourced inline 6 engine.
Wiesmann was found in 1989 and went bankrupt in 2014. Pictured is the the MF5 roadster. Customers had the choice of either BMW I6 and BMW V8 engines on the MF Roadster and Coupe as well as a V10 on the Hardcore Wiesmann MF10.
Young Engineers Sportscar or YES! was found in 1999 and started out as a dissertation. The YES Roadster is powered by either a 1.8 4-cylinder or 3.2 VR6, both VW sourced engines. YES builds their Roadster in an old aircraft hangar which gives customers the opportunity to take delivery of their new vehicle by plane.