Citroen relay - French wiring problem.

Hi, I'm new to this group, I have a Citroen relay 2011 with an intermittent fault, occasionally the engine will crank but not start, if I wriggle the wiring under the fuse box in the engine bay it would start. I've heard these vans are known for wiring corroding inside the insulation under the fuse box, I've looked but can't find any damaged wires. Today the van won't start at all, is there any way I can test each wire from the fuse box, I can only follow the wiring as far as where they go into the interior of the van but I'm fairly sure the problem is between these 2 points or is there anything else I can try?

Check the earth straps to engine bay and engine, a loose or corroded one can cause these issues. 
alternatively try moving one wire at a time to identify? 

If you have the wiring diagram and a multimeter you can do point to point continuity tests but it isn't easy. Even recognising the colour codes on the core is a PIA. You will need an assistant inside the cab whilst you are under the bonnet.

I would first look very carefully at the evidence you already have.

If disturbing the harness previously brought the fault on or off you have a very limited area to work in. Unwrap the tape from the harness if you can. You may well find hidden joints which have corroded. 

Look where the wires go into the fuse box for corrosion, especially within push on terminals, which can hold onto the insulation whilst the wire core has corroded away at the very end.

I don't know about Citroen but I had a bad wiring experience  with a VW Passat. There was an electronics box under the passenger seat. The box was high and dry but water had got into the harness to it. When stripped back of the outer wrap I found corroded joints which appeared to be just twisted together and covered with a cloth tape just inviting water ingress.  A few bits of wire, some soldering and pvc tape saw all well again.

Thanks for your advice, I have a multimeter and trying to source a wiring diagram now

Cheers. Let us know how you get on.

Present wife has an 09 relay custom converted to a Horse box, bedroom, sink, cooker etc

when the wine drinking, cheese-munching surrender monkeys designed the Relay they put the battery under the passengers feet. The method of connection is a tad weird

negative terminal is connected via and over centre lever-cam 

positive side is is via a plastic plate held onto the top of the battery by a piece of Brie

it is worth checking these connections also

if removing the battery get someone to help you as it is a biatch to shift and a tad heavy