Trailer lights repair, Ifor Williams etc.

Sorry you lost calf like that, 7 core wire from car to junction box on trailer, line of connectors in junction box for each of the 7 incoming wires, 2 wire cable then out to each light, 1 from correct row out and 1 return to earth row

Depending on the size of the trailer, you can get a proper junction box for the job used by Ifor Williams etc or you can use a std electrical IP44 or better rated junction box. I've only used the latter (IP55) when wiring lights.

In most setups the 7-core cable goes back from the vehicle socket to a junction box on underside of floor. What I do is then take a length of 7-core from the junction box to the left rear cluster and one to the right rear cluster. Each cable will have common wires - earth (white), fog (blue), brake (red). The remaining 4 wires are side dependent - LH indicator (yellow), black (LH tail), green (RH indicator), brown (RH tail). So on the 7-core going to the LH rear cluster when I trim back the outer sheath I'd snip off the green and brown - as they are for RH and vice versa for RH cluster.

In the junction box I'd do the same snips. It is then just a matter of connecting yellow to yellow, blue to blue, etc. The only additional effort is when you have front/side marker lights. They require connections to earth and the tail for that side. I'd normally use 2-core for that.

Inside the junction box I usually use 2-way and 3-way Wago connectors. However, I've seen implementations using block connectors and a long rectangular junction box which works well too.

I cable tie the wiring to the chassis where possible and only use cable clips if necessary and when I know the nail won't penetrate through the floor. Hammering in nails whilst lying under a trailer can be awkward and you will get covered in dust - so wear safety glasses to protect your eyes.

It can get complicated when you go to connect your wiring at the clusters. Some have a wiring tail for you to connect to and others might have spade terminals and you'd need to crimp female spades onto your wiring. For example, I put some Brittax rear clusters onto a piece of machinery and its wiring was black, green, red & purple/green. However a quick Google and you get conversions. When they provide a tail I would usually use bullet terminals to connect the tail to my 7-core. Reason being that if the cluster got damaged it is easier to replace than if I soldered them together. I would then wrap with amalgamating tape to protect from weather.

Another thing I do is to create a small loop of excess in the 7-core going to the cluster and cable-tie it to the chassis so that if I need to redo the bullet connections then I don't have to rewire from the junction box.

Last night I was rewiring a small IW calf trailer that got the cable dragged along road after it fell out. The factory had run 7-core from each rear cluster up into the 7-pin plug bypassing the need for a junction box. Not sure how the spliced out the wiring for front marker lights. What I ended up doing was putting a 7-pin socket on the trailer and connecting both 7-cores into the socket. So there are two other ways to wire up, although my preference is always the junction box.

In regards to the ground - should not need to do anything with the ground apart from match the pins. The ground used will be the ground on the chassis of the towing vehicle so that it is a common earth.

if you want a permanent connection that wont give trouble in years to come, solder the connections and seal with glue coated heat shrink

I have also thrown away the stupid junction boxes that IF insist on using and use soldered joints there, no more messing with multi meter at 6 am with a loaded box when you find you have no indicators

Screw connectors (block connectors) are fine. They can loosen over time, but as long as they are nipped up tight then the chances of them loosening are slim. I have a preference for Wagos as they are simple and maintenance free.

I normally use this type of junction box as I have them lying around for electrical work and usually can't wait to get in a 'proper' box:

I have soldered & used heat shrink too. A lot of it is down to preference and what's available at the time. If the joints are done correctly and the junction box is correctly sealed then it will last a very long time.

You can get junction boxes specifically for this work:

This Britax one is the type used by Ifor Williams. I'd consider using something like this if there were additional marker lamps on side/front and/or internal lights as with increased number of wires it can get messy very quickly.

Wiring up from fresh and maintenance/fixing are totally different. It is not something that I do a lot of but when I do the fault is usually corrosion on the connections/joints. Sometimes it is the wiring itself, although inspection usually shows easily if it rubbing and worn through the sheath and easily fixed.

I would not be sure why your headlights are staying on but it is likely to be either the switch or possibly the headlight relay (if there is one). I've not come across many shorts to positive but it is also positive.

I'd start with the switch and see if it is working okay using the continuity setting on a multimeter or wire up a lamp and battery to it.

Despite the poor reputation of the circuits, normally all gauges and lights work on the fiats (although usually the lights are dim)

The original headlight switch carries the full circuit current, as opposed to only switching a relay. This causes the dashboard connections to overheat and eventually breakdown.

Brewster electric in Lanesborough has a modified headlight relay kit pre-made for the Fiat's, which connects the headlights direct to the battery and then the existing loom is then only used for switching the relays on/off for the headlights.

In your case it sounds like the headlight switch has failed on, or perhaps some wires have fused together. A simple test to diagnose the switch is to plug out the switch under the dash and see do the headlights extinguish.

Those youtube tutorials are unnecessarily complicated sometimes. a simple DC light tester is a great way to start understanding circuits.