Fuel reserve sensor for SIP speedo flashes 'fuel' when tank is full

I have a clamshell sip speedo (v2). Wired up and working fine until I installed a fuel reserve tap.

The fuel sign flashes when the tank is full ONLY when the tank is actually full and is in contact with the common earth. 

my research so far:

I can make it work perfectly both out of the tank lifting the float manually and in the tank as long as the tank is isolated from the scooter body.

I have tried two new SIP taps and one OMG.... same result.

I have tried a different sip speedo

I have used a new wire connector outside the body

I have tried a new speedo control box.

 

This one has got me stumped