Originally Posted by Bigmuz7
how do you mean fella ?
Sorry B7, I should clarify.
There appear to be 3 different types of 'box' available.
1. Bypass relays for £10 - £50
These work well for the D3 but need an extra +12V supply as you said, plus they don't appear to have the resistance to tell the D3 when the trailer board is plugged in, so you don't get the trailer indicator warning on the dash or the changes to the rear suspension and parking sensors.
You can fit them to the car of course which makes the extra feed easier.
2. Plug in resistors for again £10 - £50
Again these work, and make the car recognise the trailer, but the canbus test pulse will normally still illuminate the indicators as there is no isolation.
I say normally as they have worked on some D3s, usually later post 07 ones, but not early ones like my 55 plate.
You can combine both of these, but I really believe the additional +12v feed is a bad idea.
3. There is a box available that does both jobs, and as I'm sure you are on Disco3.co.uk you'll know Character uses one commercially with great success.
But this was completely out of by budget at £300.
Besides everything I'm sad enough to enjoy solving these things, building things, and learning something along the way.
Oh, and I'm a tight sod too.
It's just the way I am