I think you have the wires connected in the wrong order.
Is the socket new and you are using existing wires?
Or did you connect both new wires as well as new socket?
I have a Mk6, but i suspect it’s pretty easy on yours to take off the rear lighting clusters.
On mine, the tow hitch wiring for some weird reason is hooked up to both left and right clusters.
But i think it should really be just off one, in mine would be the left.
Note: left and right are based on you sitting in drivers seat and looking forward.
To remove the cluster, In mine you open the rear doors, and there are two plastic wing nut type nuts holding the cluster on.
Undo them.
The plastic plus the lamps and the socket they fit into can be prised out.
Pull that out, should be enough wire to have it come out.
Then you should be able to see what wires lead to what lamps.
Top is two filament: moderate bright for rear light, bright for braking.
You can figure out which is which by having assistant put in reverse, put fog lights on, indicators etc.
So you now know what wires are what for most of the system.
The tow hitch wires come off this.
You can see what wire is what if the colours don’t make it obvious.
Remove the lamp/holder board and you will see the metal connectors that the lamps contact.
In order to figure out what wires are why if the colours don’t make it obvious, you need to use an ohm meter (with ignition off) to see whatvwire is what to the wires going to the tow hitch.
If you cannot get a multimeter/ohm meter, then you could get some wire and temporarily attach a spare bulb to a given wire on the tow hitch.
And the other end of the bulb you attach to something you know is ground.
Then go through in sequence the various lights: rear lights, brakes, hazards, fog etc.
When your bulb lights, you know what wire it is.
Do that for all the wires you don’t know what they are.
Label carefully when you know.
One will be the ground, so when connected to that the lamp will not light.
But that really should be easy to see: is going to be green or black in 99% cases.
Once you know what wire is what, the socket should have come with a pinout that says where the wires go.
If not, google 13 pinout tow hitch and that will tell you.
Google images will be most obvious.
https://www.google.co.uk/search?q=13+wi ... 66&bih=922And you really should have a multimeter: i would buy one where you are.
Don’t need anything fancy.
Amazin will send you one by tomorrow if you join prime for a free intro.
[url]https://www.amazon.co.uk/ULTRICSÂ-Digital-Multimeter-Voltmeter-Ammeter/dp/B00TM0W8ZY/ref=sr_1_4?ie=UTF8&qid=1534115321&sr=8-4&keywords=Multimeter[/url]
That would be ok as a simple one.
Or else Halfords.
I think if you don’t have a multimeter you shouldn’t do any wiring.
It’s a bit like trying to fx something with a knife, rock and pliers instead of having screwdrivers, hammer and wrenches/socket set.
Sure in an emergency you can make do, but it doesn’t do a good job.
If you screw up, all you will do is blow a fuse.
Will be one in the passenger compartment.
Owners manual says what is what.
And again, if you work on electrics, you need to be comfortble checking/changing fuses etc.
Best is get a buddy who knows to help at first.
Hope this helps.