[11B]

I'll bet the brakes on the trailer are applied by tongue pressure to the hitch. Check and see if you have a sleeve that the trailer tongue fits into then runs the rest of the distance( in a larger sleeve) to the hitch. It's been my experiance (20 years selling trailers) that this is a demand build feature on all small trailers weather they are set up for elec. brake systems or not.

Part of the reasoning behinde this that a lot of folks will pull barrowed or rented trailers without the lights hooked up or have the abitity to hook them up.

This feature would explain why your trailer brakes apply when the retarder kickis in. No power required.