Trailer Wiring

Having just installed a tow bar on my E46 330d Touring, I am having trouble getting the trailer wiring sorted out. I have a relay unit in the system, so as yet I do not have any trouble
with the bulb failure unit. The problem that I encounter, is that when I connect in the power to the relay unit, I get a warning beep from the car. I have connected the power supply to the trailer module, immediately after the main 200 amp fuse black, and then installed a secondary fuse for the trailer only.
Any one know how to get this sorted?
PyroJames Where I like, when I like, in whatever I like to wear.
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