Well, I found my answer by reading the FAST XFI instructions. In chapter 21.0 on "Aux. Outputs" it is explained how to make a shift light function.
The only thing I'm not sure about is if there already are some provisions to plug a wire into pin "B1"... Am I supposed to check for a connection into the harness or the FAST unit itself?
Other than that, it appears pretty simple.
Claude.