I had a problem the other day when I conected the trailer plug to the H6 .when I went down the road then stood on the brakes no ABS ? ( my trailer does not have brakes ) I got out the car disconected the trailer pulg ,whent down the road stood on the brakes and every thing was Ok . very weird ! need to sort the trailer ,may be earth problems ?

Did you lock up any of the wheels? As I understand it, ABS will only activate if it detects a difference in wheel rotation. With the weight and momentum of a trailer, you may have not reached the point of lock up.

mate I stood on them some thing was not right the brake fail light came on as well ( forgot that bit it is late over here in Perth )

when I pulled the plug out the car was back to its self again.

The abs does this have some thing electrical it has seen with the trailer pluged in .??

I have a trailer for our 01MY Ltd, I know the ABS works with the wiring hooked up because I had to hit them hard for a short stop once (lucky the trailer was empty so no load shift). I would check the electronic switching box for the wiring first, if the box has to translate the brake and turnsignal lights into one trailer light (mine does so I assume you don't have a separate setup for turn and brake lights on the trailer). Could be it's sending feedback or something into the electrical system. I would double check ground (earth) connection, you could hook a temp wire up direct from the lights back to the car for a test.

Is this an aftermarket setup? And does it use the plug supplied in the back of the car?

If so then maybe you should try a different wiring harness.

Hope this helps you out.