Technical Front wing indicator repeater stopped working

[toyogran]

After a year of trouble free motoring I've had an annoying problem getting an mot on my 2005 2.8jtd Adria motorhome due to a failed front n/s wing indicator repeater lamp.
I've had the lamp unit out and checked the usual things but all looks in order and corrosion free. After checking the voltage when the indicator is on the voltage is very low compared with the o/s so suspect a wiring/connection fault further up line.
After taking the air filter etc out I can see that the feed wires disappear into the main loom that is attached to to n/s inside wing and don't fancy stripping that to see where the 2 wires go! It seems that at the lamp holder end one connection is "earthed" and the other receives the pulse voltage for the indicator.
Anyway to comply with the mot I took a feed from the front n/s headlamp unit indicator and wired up the repeater lamp from there. All seems to work ok and repeater works like it should.
Has anybody experienced this problem or any ideas where the original feed comes from and what the likely cause may be? Anybody got the relevant wiring connection diagram for the wing repeaters?
Any ideas or solutions other than the one I've got at the moment appreciated!
Belated happy 2017 Ducato motoring.

 

[Badknee]

I realise you have it sorted (well done) but I would suspect a poor earth. Low voltage you said but were you testing both + - at the bulb holder or just the + with a known good -?

 

[toyogran]

Thanks Badknee, yes checked it out with a good earth and the same. Have since got the wiring diagram from Elearn and going to check out the connections at the various junction boxes from the dash (if I can find them!).
Sure it's something simple but finding it's a different matter!