Take 2 - just done a long reply and my bloody laptop jumped back before I post so here goes again.
Can't give you a defined answer on your 1.9D tank lift pump but what I do know is sometimes manufacturers use common wiring looms for both petrol and diesel models. I suspect this is the case with your car. The two thinner wires are for the fuel guage and the thicker wires for the tank lift pump. As you've found, no pump on the diesel, but would have been one present on a petrol.
Going by my 05 1.9JTD, Fiat would have changed this and fitted a tank lift pump which is used for initial priming of the fuel system only. Once started up, the main pressure pump takes over and draws the fuel up. I know this, because I had a problem with my tank lift pump relay in the electrical circuit. Prior to me finding the fault, the car wouldn't start unless I hand primed the fuel system and once fired up, the pressure pump would draw the fuel up and the car ran fine.
As for the 1.9D being gutless, as it always been the same, or have you just recently experienced a drop in power? Without being disrespectful to the 1.9D, by comparison to my 1.9JTD, I find my car gutless a low revs, but once the turbo boost comes in approx 2k RPM, I find there is a lot more umph about the car.
I didn't know what performance to expect from the Fiat 1.9 diesel, but having found it's a steady plodder, can understand why the 120 1.9 multijet would be probably the best of the bunch for the older diesel Doblo. That said, DPF issues to be considered in the equation.
