Under the hood on the passenger side near the windshield and the hood hinge, there is a flat and large plait wire connect to the body. This wire is disconnected at the other end. Where it suppose to be connected ?

 

Sounds like a grounding wire to me.

 

I believe that it connects to the motor mount. It is a grounding strap.

 

Ok i will reconnect it but what kind of situation can happen if not connected ? A fuel gauge not working for example ?

 

A bad or missing ground can cause all kind of funny problems. I don't think the gauges if its a motor ground.

 

If multiple other ground sources from engine is ok you may not see any problems at all right now. But there are products for other cars that add several great grounds to the egine and trans to lessen electrical loading and have been shown to increase horse power.