Function output conflict Bachmann Class 66

[blanman27]

I am in the process of wiring all my loco's to have independently switchable & directional lighting (cab lights, tail lights) and have a strange problem with my Bachmann class 66's. When I connect the tail lights to a separate F0 (Aux3, Aux 4 etc) it works, but switches the front white lights off! I use Zimo sound decoders, in this case MX644D, but I have not had this with other loco's I've converted so far (class 37's, class 47's, class 57's). I have tried different decoders including non sound (6 function TCS) and it's just the same result. This leads me to think the issue is with the PCB. Anyone got any ideas what might be causing this? Suggestions greatly appreciated. Cheers, Paul