I don't think the X5 has a built-in trailer brake contoller in the system. I thought perhaps I just wasn't familiar with it, and searched a bit. I found LOTS of entries on the 'net about people installing Tekonsha Prodigy brake controllers in X5s, and I'd think they wouldn't bother if BMW provided trailer brake controls.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill