The ground lug is maybe just under or behind the ECU
It's got 4 wires on a metal plate that bolts onto the body.
If you cant see it, then try unbolting the ECU bracket so you can pull it away
As I remember it's easy to find. Once you find it clean everything shiny with a wire brush and goop it with neverseez or conductive grease. Inspect the
crimp lugs- they can get loose and full of corrosion- If in doubt
crimp a new spade lug and use dielectric silicone or solder
(I like soldered crimp lugs since they don't fail intermittant)

This is why the Volvo wiring Greenbook is essential... It gives locations
for everything. You don't have a chance with Chiltons or Haynes
with their generic schematics and no locations for stuff.

Bill