Sidelight electrical issue

[Blackshuck]

Got an intermittent problem with the sidelights on my '04 2.0hdi Berlingo based Romahome camper .
The front & rear offside sidelights + numberplate light & hazard warning ; central locking & rear window heater button lights are all working occasionally - seemingly at random . 
Sometimes when I turn on the steering column light switch , they all come on for a second & then go off , only to come back on later . Other times they come on as normal & then randomly all go off .
The steering column Comms unit was replaced a few years ago .
I gather that the sidelight fuses work diagonally opposite lights , so would the fact that my problem is affecting both front & back offside lights suggest it's not a fuse issue ?
Also would a Comms unit light switch issue only affect only the offside lights rather than all of them ?
Do all these lights have a common earth ?
When I turned the lights on this morning , they all came on as normal , but then immediately went off again , only to come on again a few miles down the road .
Doesn't seem to be linked to vibration , i.e bumps in the road . I live in mid Wales , where it has been very wet lately  , with high humidity levels .
Headlights ; brake lights _ indicators + all other interior / dashboard lights work fine - as do the nearside sidelights .
Anyone got any suggestions / advice as to what's going on ?

Update - just started it up this morning & tried the lights - front & rear offside light ; numberplate light & dashboard switch lights mentioned all dimly flashing on / off every 5 seconds ...... ???
Is this a diagnostic message for something ?

[my mucker]

Hi Blackshuck ,i dont know a lot about electrics but i imagine an intermittent fault is the hardest to detect !! Personally i would check all fuses for the lights concerned and pull them in and out to establish a better surface contact !! I am sure there is someone on here will give you more advice ,hope this helps !!

[my mucker]

Hi blackshuck,if you go to members tutorials and how to's you'll probably find what you're looking for there !!!

[brodfather11]

I'd look at the rear lamp clusters , wrong bulbs can send a feed back to the other lights in the other side , remove one sidelight fuse then see what happens then try  with the other fuse removed.

I cant remember if the brake lights and rear fog lights have a separate fuse  

Changed any indictor bulbs lately and with the proper fitting ones ?.Orange ones with offset pins.

Side light bulbs can also be fitted with the wrong ones

[Blackshuck]

Had a good look at it over the weekend & finally sussed it out - the problem was with the number plate light unit ( on my van , a Jokon K307 ) .
The steel baseplate acts as the earth conductor , with a wire going from this to an earthing point - this was OK & was earthing out when tested .
The issue was that the live feed connections for the bulbs are also fixed to the baseplate although are ( or should be ) insulated from it .
On mine , on one connection , the insulation was not 100% , allowing a slight & intermittent short to earth , which was then causing the BSI unit to cut off the live supply to the circuit involved .
The lightbulb ( !!! ) moment was when ( with the lights switched on but not working ) , the feed to the number plate unit was disconnected .
This stopped the short , the BSI was happy & all the other non functioning lights came on .
Managed to repair the number plate backplate , so eliminating the short & now everything seems to be working as it should .......

[Gryffindor]

Great detective work. You should set up in practice.