As it says in the title really.

I've brought a second-hand tow bar and electrics to fit onto my V40 and was just wondering if anyone knew of any wiring diagrams or guides to fitting?

Thanks for any help you can offer.

You need a relay unit which goes between the tail lights and the 7 pin trailer socket.
This is to ensure that you do not blow the main lighting circuit and that the bulb alive circuit will continue to function.
You should receive a wiring diagram with the relay unit.

Hope this helps.

You don't have to use the relay unit. There is a way of connecting in directly so that it doesn't upset the bulb failure detector, I did it on my 1997 V40, but I can't remember how, it was years ago!
Either you can use a split feed to your 2 trailer brake lights and pick up the appropriate sides of the car wiring near the lights, or you can take the tapping point upstream of the bulb failure detector so it doesn't influence it. I think that's what I did, but I can't remember how far you have to go.
Sorry this is a bit vague, but dig a bit in this and other forums or on Google and I'm sure you'll find a reference to it, that's how I found it.