Wiring external antenna to wf2830

[blightfoot66]

Hi, I have a 2012 W639 Vito which has poor radio signal. Was advised that it was generally poor from the in screen antenna and the best solution was to install a roof nounted aerial. I have done this, but now have to attach the power wire from the aerial for amplification somewhere. I think there should be a particular wire on the rear loom of the wf2830 radio to connect this to, but it isn't obvious. Has anyone else fitted an external aerial and know where to attach the amplifier power lead? Any advice greatly apreciated