Another way to operate tail lights separately is to use one function output to operate a miniature relay. It needs to have a 12v coil and be the DPDT type. Cut the wire to the tail light at each end, wiring each end through one changeover contact, and mapping the output onto the desired function button. If the contacts make with the relay operated, the lights will come on directionally, and if the contacts break then the loghts will go out. It's down to personal preference which you like. On my Bachman. Class 40, operating the function button turns the tail lights on, so the relay spends most of it time unenergised.
Suitable relays come in tubes of 40 from Rapid Electronics - no connection other than as a satisfied customer.