This may be related to all other electrical faults am having, i.e revs dropping when headlights switched on or when brake lights come on:

I think I may have a issue, (well to be honest I have a few, but just want to sort this out first)

When I disconnect my 2 earths on back lights, 1 for each side, the lights are still working,

For instances, my stop and tail lights has 3 wires, 1 earth 1 for stop lights 1 for tail lights, if i disconnect the earth, the bulb still lights up.

It is not caused by bulb earthing through light houseing, as the housing is all plastic.

i remember years ago on the old escorts orions the multi plug connector used to corrode causing light issues not sure if this problem still exists on newer models but might be worth a look in that area also with known spoiler and rear light water ingress issues on the focus would be worth checking all the earths at the rear .

i can only find 2 earths in boot, 1 for each light and they are both perfectly clean.

I have noticed, if i take out both stop and tail light bulbs, and switch on headlights whilst engine on tick over, revs stay stable, but if i put in rear brake and tail light bulbs, and then switch on headlights, the revs drop again.

I have tried a new rear wiring loom, but still the same.

I am now thinking about rewiring it my self, i.e new wires from brake pedal switch to brake lights, new wire from headlight switch to tail lights, etc etc.

how much are the revs dropping as rev drop is normal when load is put on alternator is it going to the point of nearly stalling or just a bit of a drop

its quite a noticeable drop, but not to point of stalling, but is a pain when coming to a stop and press brake and car starts shaking, or sitting at lights with window wipers on intermiteint, and every time wipers start, engine shakes, and only stopes when wipers park them selves.

It is really baffling me now, the alt output all seems normal, alltho i am considering changing alt to a 105 amp instead of a 80amp

Have you tried the main earth under the bonnet

I have replaced the earth lead on left hand side or engine from body to engine, and added new one on right side.

All so put extra earth lead in from earth on battery on top of strut mount, and one direct from battery to engine, and one direct from battery to alt body, and new one from battery to gearbox.

And new + leads from battery to alt and from batt to starter, and new one from batt to fuse box, and they are all nice and thick cables.

If there is any testing you need me to do, let me know and i will go out and do it, i really need to get to the bottom of this.

mate why dont you take it to ford the pcm may need an update to sort the revs issue my old mk1 and a few others did also it may have nothing to do with the wires the parts that effect the revs mainly are sticking or faulty pcv valve or weak pcv valve spring ecu diode faulty and needing replaced the pcv hose collapsing under acceleration pcm requires values updated faulty temp sensor faulty maf sensor split vacume hose all these can cause the issue with load applied to the engine on top of that you have other issues ive been there done that spent a lot of time and money trying to fix it and in the end went to ford a pcm update fixed my issue and a new pcv valve and hose.

many mk1 owners have never got to the route of the problem the ecu diode for example is a well known issue that requires specialists to test and replace the diode.

having dealt with a similar issue i can tell you that you could rewire that whole car and end up frustrated some have with all new earths