One of Caravan Chronicles long time readers and supporters, Colin “Snowy” Snowden, has sent me an excellent article he has written covering all aspects of upgrading pre September 1998 UK caravans to the 13 pin Euro Plug.
It’s very comprehensive article and goes into detail about the various problems and colour codes that were used and cover some of the changes that are required, habitation relays and details the steps required to actually do the upgrade with steps on fault finding.
If you have a pre September 1998 caravan and are thinking of replacing the original 12N/12S wiring with a 13 Pin plug, it’s an essential read.
The document can be downloaded and viewed here: Upgrading to 13 Pin
Colin has included contact details in the article, but if you have any questions that the answers might be helpful to others as well, you could also post them in the comments section below and Colin will be able to answer them.
Caravan Chronicles 
Sorry, forgot to mention that this new version of the article now covers the upgrading of all caravans not just pre September 1998.
Colin
Re Pin 12 for trailer sensing by the ECU.
I have a 2014 Hyundai Santa Fe which has trailer stability built in.
You may be interested in the response I have received from Hyundai when I asked them how the car detected a trailer.
“The vehicle does not need to know that a caravan is attached for it to activate.
The system is built into the existing Electronic Stability Program and utilising the same sensors (steering wheel inputs, wheel speed data and the lateral/longitudinal yaw rate sensors) to determine if the trailer has begun to snake or lose control, it is able to do this as the swaying motion of a trailer or caravan is unique compared to other situations that you may encounter whilst driving. In these situations the yaw rate sensor will detect the vehicles movement but without any corresponding input coming from the steering wheel to account for this movement, it then knows that a swaying trailer is the most likely cause and act.
It will then apply the brakes to individual wheel to counteract the instability and control the snaking motion of the caravan. However the system can be overridden by the customer if they try to control the vehicle themselves by using the steering or braking system excessively, at which point it will switch off as it assumes the customer has taken control. Basically if this happens you need to trust in the system to arrest the trailer instability itself without intervening yourself to any real degree.”
Amazing technology.