Hard to diagnose without seeing it, anyway, have a look on the back of the gauge cluster, #1 is a 4g93 and a #4 is a 4g15. They are not interchangeable, just to make matters more confusing the MR tacho is different again, and it is just as Donki said. This is a number 4 cluster so as you've mentioned it should be for a 4G15.
First off i see you added another earth, DID you connect it to the tacho earth point marked on the back of the dash?obviously that did not help, so moving on, do you have an LED test light (the normal filament type draws too much current and wont light up enough) to allow you to test for fast pulses. Yep, back of the cluster which read "Tacho Earth" then i grounded it. I do have a test light. That's a good idea.
Mind you to do all this you will have to isolate the tacho pulse wire in the back plug going to gauge (tacho pulse wire, if one is fitted) i cant remember to well but i think there is a connection point on back of gauge for tacho pulse (I don’t mean the plug)’’
If not cut existing wire in plug that is pulse wire, isolate it/use only to join input wire from following sources. This is where i'm slightly confused, there is a track on the cluster marked "Pulse" I'm assuming..... This is for the tacho??? - It doesn't state tacho however. Though on the actually plug itself there is definitely a pin for this.
You could try blue connector (black and white wire) above wiper assembly in rear firewall, is it plugged in or not? If that fails which it prob will, move to next step. Blue connector was not plugged in.
Splice in pin 58 at ECU and try , then if that is a dead end and doesn't work, and this is a long shot by the way, try splicing in wire on output pin from ECU pin 89 (crankshaft sensor) i read somewhere once, might even have been on this forum, that this works a treat when all else fails, i have never tested it or tried it, so can't tell you if its correct or not. (be carefull don't connect wire to circuit marked pulse on back of gauge, i'm pretty sure that is the speed sensor otherwise you might fritz the sensor) then you will have two problems, check fuse 10 as well, works off interior light circuit, guess if lights work than fuse is good. Ah right, you mention "Pulse" here so looks like that's not for the tacho? where is the actual tacho signal markings on these clusters? - See my pics added.
I'm not sure what harness you have but later gl Gil had tacho harness along with mirages, it unusual for the tacho to jump around like that when you plug in a non tacho harness on an early model(but 4g93 and 4g15 are different, along with MR tacho, so maybe that is the reason.