My trailer lights work fine on my old van 06 but on my new car 10 will cause the indicators to stop working altogether. On my 14 reg car the trailer causes the park lights to fail altogether

Here ya go.

possible that the trailer plug is not wire correctly

it is also possible that the trailer has bulbs and the kia uses led units which will throw out the electronics
you may have to talk with an accredited auto electrician as there may need be an adaptor or blub change in the trailer electrics

typical UK reg motor they corrode like ?-?-? ,i suspect that your horn has stuck up with corrosion and the indicators is the hazard light warning switch faulty .But i could be wrong and often am but check it out anyway.I try not to repair UK or northern europeon vehicles anymore as i spend more time cutting off rusty bolts and seized bits than whats the cars worth compared to spanish vehicles that can be 20 yrs old and i just whistle and the bits come undone

firat of all check and make sure you have a real good ground or otherwise you will lose current to another wire and this can short out and cause the lights to malfunction.

White: Ground
Brown: Running
Yellow: LT and Stop
Green: RT and Stop

This is the wiring code for the 4 wire trailers and
Wire a round seven-pin hitch, typically sold without a wiring harness, to the vehicle observing the following color codes: Run a white 10-gauge wire between ground and terminal one, run the blue 12-gauge wire between the electric brake controller and terminal two, the brown 14-gauge wire between the taillights and terminal three, the black 10-gauge wire between the permanently live 12-volt supply and terminal four, the yellow 14-gauge wire between the left turn/brake light and terminal five, the green 14-gauge wire between the right turn/brake light and terminal six and the purple 14-gauge wire between the backup light and terminal seven (in the center).

You blew a fuse. Check the in cab and engine compartment fuse box and replace burnt fuses. It's possible that you have too many lights on the trailer. This would cause a multiple amp draw on the truck pulling more power than the typical 10-25amp fuse supplied for trailers.

The trailer lights on your van should be fused independently of the vehicle lights if you have the factory wiring. There is a fuse junction block under the hood [should be in the left corner somewhere] and in that you should find the fuses for the trailer lights and other functions. This is from memory but check fuses #5 and #6, [10 amps fuses] and see if they are blown. I could be wrong on the numbers so you might have to check them all. Usually its the trailer that causes these fuses to blow so if you find a blown fuse replace it and make sure it doesn't blow again by testing all the lights with the trailer unhooked. If that's ok then hook up the trailer and check again. If it blows with the trailer hooked up then you probably have a short circuit in the trailer wiring somewhere. Good luck, hope this helps!

is the trailer attached to the ball on the van?? could be a ground problem,,run a jumper from hitch to trailer see if that works,,

Saves having to use a supplementary socket
01 Yellow Left Indicator
02 Blue Fog light
03 White Earth (35amp)
04 Green Right Indicator
05 Brown Right Side Light
06 Red Brake Lights
07 Black Left Side Light
08 Yellow Reversing Light
09 Green Permanent live
10 Blue Ignition live
11 Red Earth (35amp)
12 Black Coupled Trailers
13 White Earth (35 amp)

Have you tried a heavy duty flasher? Also, if your signals & brake lights are 2 different bulbs, as in amber for signals & red for stop, then you will need a small in line converter to separate those 2 as most vehicles use same bulb for signal & stop.

Look under the dashboard, next to/above the park brake pedal, for the interior fuse box. It has a black plastic cover. Remove the TAIL fuse, and see if it is blown. This should get you fixed up. Hope this helps, and Thanks for choosing FixYa.