Rain strength not adjustable

[Gaudin de Beaufort]

I cannot set the rain strenghth in the ext rain app. The value remains 0.0 and does not respond to mouse input. However, the whiper setting does respond. Windscreen FX says "is used to draw rain drops". Even when "sol automatic rain" is ticked, i have no rain on the windscreen. I have chosen a wet track and weather is set to "rain". Does anyone know a solution?