Can you give me more guidance on where to attach the earth? How do I attach it to the black yellow cables?
If you are going round the A pillar then the closest viable earth point is behind the cover on the side of the dash only visible when you open the door but access is tight; the simplest place is probably by removing the cover beneath the knee shelf and finding one there. Alternatively the cover around the bonnet release lever unclips easily and there are many options behind that.
If going for a central dash top mounting then removing the radio and gaining access to the mid point of the dash is possible, plus there is easy access to all suitable power & earth. Search Legend-ary's "Fitting an aux-in socket" guide for a good pictorial of radio removal. Drilling a neat hole in the dash of a Mk1 is probably OK these days, but I would hesitate in a Mk2/3.
Cables routed to earth (chassis ground really) are 95% black / 4.5% black-yellow / 0.5% brown-black.
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TG