More than likely it's nothing you did. All the connections for the EL lighting are inside the cluster and protected by the bezel. Most likely the EL connection to the fuel gauge face itself is coming loose. Don't try to solder it! The only way to fix it correctly is to use 2 part conductive silver epoxy. If the EL needle still lights up, that is definitely the problem. If you have lost both the needle and face lighting, could be a bad ground between the gauge and the cluster housing. Rebuilding/repairing those EL clusters is the main stay of my gauge business. My contact info is on my website, www.thegaugedoc.com. Feel free to contact me if you have more questions....
Mark
the gauge is grounded to the cluster housing by two small metal tabs on the underside of the gauge, in this case the fuel gauge. First thing to check and see if the speed nuts that hold the fuel gauge to the cluster housing are tight. If they are, the remedy involves taking the instrument cluster out, tasking off the bezel, taking the fuel gauge out of the housing and cleaning up both the underside of the fuel gauge and the housing itself with a wire wheel. There is probably some corrosion on those two tabs and it lost contact while you were getting the speedo cable installed. If the rest of the gauges still light up, it is an isolated problem with the fuel gauge ground.
Mark
Based soley on the fact that you just hocked up your speedo cable which i guessing is mechanical i would say you have bumped a wire join and it has died. Check all wires are attached and are in good condition. If this is the case attach a test globe to the fuel tank gauge wire and turn the lights on. If it works its your gauge if not start the unbelievably fun job of checking the wire for pinches and breaks Good luck
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Also check all the earth points on the car, poor earths do all sorts of strange things
Any more news on your gauges?