Good question. I had to look it up as I prefer it either off of on full time and use the LED color lights for alerting. This is especially true for me when on battery power, in order to save energy.
Easier to Read Pro-651/652/106/197 - WS1040/1065 - PSR 500/600 Digital Scanner Manual
1. Select the "CONV Objects" tab
2. Now on the right side under "Alerts" you will see an option menu for "Backlight"
3. Select the conventional frequency you want to trigger the backlight on when the squelch breaks and then select On or Flash, whichever you prefer.
If you want all of your conventional frequencies to turn on the backlight, there is not a global setting that I am aware of. Simply select the top most freq, hold down the Shift key and then select the last freq. Once they're all highlighted select your "Backlight" preference. I have not tried that, but it should work.