You should have 5 volts on the blue wire at the post and gray is ground, eventually. It's splices in and shares a grounding point with a few other things and I'm not sure where the ground point is on a Mirada. If your interior and rear lights all work, the body ground point is good and the wire from the sender is broken (assuming it doesn't have continuity to ground). The sender is a variable ground. I can't remember exactly off hand right now but it's something like 10 ohms (full) and 90 ohms (empty). 12 volts goes into the gauge voltage limiter and comes out as 5, runs through the gauge and continues on to the sender. What the green wire is,,,,,,,,,,,don't know, sorry.
Check the voltage at the blue wire for approx 5 volts and check for continuity to ground on the gray. If you ground the blue wire, the gauge should peg on full.
If the power and ground at the sender are OK, it's most likely the gauge since the sender is new. New just means new, doesn't necessarily mean good but since the gauge has never worked you can assume the sender is good. About all that's left after that is to try grounding the gauge right at the cluster, which may be easier said than done, and seeing if it pegs on full or not. Do not pull the gauge and bench test with a 12 volt battery, then you WILL have a bad gauge.
