First, assign one of the BRT#_Ctrl as described: https://simvim.com/svc_brightness.html
That's all you need to control your hardware radio displays.
If you need to change the brightness in the virtual cockpit screen, open the "Input Options" plugin menu, rotate the knob to see reaction.
Then select needed dataref.
For the Laminar B58 it is standard "instrument_brightness" with index 5.