You need the entire harness as shown in JCL's post. This will put the 7 pin round connector next to the receiver. You would use the adapter you've pictured to plug into the 7 pin round and adapt it to the 4-flat plug you have on your trailer. I use one of those adapters too when pulling the jetskis or a small Uhaul trailer. But the harness set you get from BMW includes the correct plugs to plug into your vehicle's wiring harness without butchering it up, and it has an electronics box as part of it which really makes everything work right. Remember, the wiring in an X5 is hall-effect, in other words some of the wires in the system do more than one thing, and the light control module in the right front foot well controls the pulsing of the signals through the circuits. It's very easy to muck it up.

BTW- nice looking setup- white X5, white bike, and even a white trailer. Nice.

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'04 E53, 4.4 Sport
'97 E39 528i
'86 911 Carrera, track car
'96 BMW R1100R