@GermanYager200i, welcome here.
This injection based fuel and start system on your 200 gti doesn't bear very well with voltages under 10 V DC, means it will not fire up if voltage drops under this value.
So, do find the culprit:
-ignition off
-take a multimeter and put it between battery+ and cable
- ignition on
- multimeter on ohm
- IS there is current flowing?
THIS Yager/Dink have a charging socket within the seats undertray(pls see pics), sometimes this and illumination is emptying the battery, if its switch is not closed completely.
Take the bulb out, check the 12V socket.
Pls give a feedback to learn.
EDIT: CROSSBOLTS say got me to think of this ground points at your startermotors rear end.
A corrosion there or at your fuses connectors might lead to battery consumption.
I had at my YAGER/Dink a blank cable within rear harness, close to my LH shock, very hard to find.