Pin 5 LSM . You could check at the Lsm plug connection is good LG/U . I presume you have a helper or pushed the brake pedal down with a wedge to check current. Left hand side of boot is a double earth , sometimes one works and the other does not ( if crimp solder on one wire not good ) Remove +clean). Does the reversing light work that side ? Even if ok the other earth may be duff due to bad solder to crimp. You could check continuity from pin 5 LSM to pin 4 LGU on the plug / light unit. I dont know if the lights are bridged inside the boot or not ,as there is a header join, but someone who has installed trailer lights would probably know ! ...
