It may be the 'enable lighting output direction' CVs, 57 and 58 on the decoder I'm looking at now. I'll read some more and then test, and report back.
Table G. CVs 57-58 Values Lighting Output CV Value
Headlight +1 to enable
Backup Light +2 to enable
FX3 Output +4 to enable
FX4 Output +8 to enable
FX5 Output +16 to enable
FX6 Output +32 to enable