On 84 there are a couple key ones.
Two on the passenger side cam cover, one just over the "O" in PORSCHE, one just above the space between C & H --- one above the O is hidden behind/under the valve that is part of the air pump emissions, you must remove it and some of the hoses (easier than it looks) to clean it. These are the most crit grounds on the car as all the engine electronics ground there.
Another crit ground is under car, linking engine with frame, pass side.
I'd replace the neg bat ground strap with a similar flat wire weave strap. We've seen at least a dozen of these straps fail for corrosion reasons, failures hidden by plastic cover. Trust me, it needs to be replaced.
( Of course the pos bat post and the little red positive on battery need to be thoroughly cleaned and carefully tightened.)
Inside cockpit you must clean the two sets of grounds located above fusebox. Fusebos needs to be removed, cleaned and re-fused with the german fuses (do NOT use BUS brand, they don't fit). Inspect back for issues.
Remaining grounds are in cockpit under steering column, in eng compartment at each side of radiator, and inside car -- behind passenger rear quarter interior panel --- PITA to reach but it does ground the fuel pump. The electrodes are brass, not plated.
I'll post a link for 84 Euro that has most of these grounds identified for you. (Euro has an engine ground near the PS cam cover that yours wont have--- its the first one I deal with on post # 7 of the link; also antilock grounds yours won't have)
Here is link, I also cleaned 14 pin connector in this diatribe, and replaced some fuel lines and other wear parts.....
Ground Cleaning, 16V Euro