Scratching head over lighter plug wiring (Sirocco Fan)

I'm just installing a Sirocco 12v fan in the RTT. I've wired it to a cigarette (socket) plug so I can also mount it on the side of my canopy if required.

I have wired it to the common inlet from my house batteries, this inlet runs the led light (works fine) and a 12v blanket (works fine). All wiring and fusing is adequate. I have the female socket wired via an anderson connector, same as the wiring for the blanket. The wiring to the socket connector shows 12.75v at the pins (checked the wiring to the light and blanket, same...). However as soon as I plug in the plug from the fan it shows 1.95v. Tried a usb outlet (which glows blue when power available) and it shows 4.2v!?

Both the fan and the USB outlet work fine from the other outlets attached to the same system, just not this one in the RTT. So I purchased a second outlet... same result!.

With the wiring oversized for the use (fan draws very little) and all fuses intact (and working on other outlets) I cannot figure what is going on.

Any suggestions?

Cheers,

Mark