Was thinking like this
Don't mind the dog hair.
The button doesn't need to be large.
Trying to think of a way to do it without putting the switch in the linkage.
But at the end of the day a switch that is pressed by the linkage when in first gear is probably the best way to do this.
The other way would be some kind of arduino/microcontroller, that reads canbus and sends out a voltage when you're in first.
MotorvateDIY could probably do it, but probably does'nt have time to.
Also, I would never opt for this canbus solution when a mechanical solution exists (the linkage pressing the button directly)
/shrug