While not helpful with your issue I have a question. Twice recently I have jumped into our mini ( large left tank Cooper S right tank ) 85 1000 E. The fuel gauge has been very good for the last 100,000 miles. When the needle lines up with the E stripe you are out of gas. The time require to use from 1/8 tank to empty is very quick. The problem is twice recently I have cranked up and started out someplace to notice the needle well left of E and not moving. Each time I stopped checked connections at tank and put $20. in the left tank. Get back in car and gauge is acting normal. I could also tell there was more in the tank than $20 even after the tanks balanced. I'm going to put a gallon in the boot and se what happens next time it gets low. I keep new sending units and tank seals. Steve (CTR) 

With both sending unit wires connected at the sender, disconnect the green/black at the gauge.  Connect a multimeter between that wire terminal and chassis ground.  With the meter set to measure Ohms, use a coat hanger through the tank filler neck to lift and lower the float arm.  You should see that nominal range of about 270-240 Ohms with the arm all the way down and around 30-33 Ohms with the arm fully up.  If you find that range, the sender and its wires are OK.  If the resistance is not changing and within that range let us know and we can suggest other tests.

I am decent with wiring gauges but I wasn't sure if there was something I was missing,I thought the green/blk was the sending unit signal wire but for some reason the gauge isn't reading so maybe it's an issue somewhere between the gauge and tank, time to start Tracing back lol

The green/black is the sense wire and you will connect it to your new gauge.  The black wire on the sending unit must be left on the sender with its other end connected to the car's chassis for the sending unit's ground connection. 

To work with the Mini fuel sending unit you need to have bought a gauge calibrated for 240 Ohms = Empty to 33 Ohms = Full.  That is an industry standard range and it is the one closest to the range of the Mini sending unit after 1964.  If you bought a gauge that uses a different resistance range, exchange it for one having the 240-33 Ohm range.

Do not connect your aftermarket gauge to the light-green wires.  Look for dark green wires without color stripes and splice into that for power.  Ground and the internal lighting should be easy to sort out when you start working on the dash.

Mk3 car

I have a digital gauge set up in the car,mini harness powering the chassis etc and a minitec harness powering the engine.

The factory fuel sending unit has two wires,one black and one Green/Black...I have the green/black wire I cut on the plug where the stock instrument cluster would be...Is this wire my signal wire for the gauge? I am looking at my Haynes manual and it shows a light green wire as well and it's traced back and linked to the voltage stabalizer?

I know my auto meter gauge has a power source,ground,signal...and then the lights. ..my digital panel is powered up...do I just attach the green with a black stripe to it for signal or am,I missing something.