There are two ground wires to engine. One is a large one going straight to the battery:
View attachment 47012
Sometimes the connection to cylinder head gets rusty, so take the time to remove the bolt, wire brush the terminal (as well as head and da bolt) and reattach to cylinder head. Sometimes the battery terminal needs cleaned. Sometimes the battery cable (negative) can go bad over time (or corroded, etc.).
On back side, there will be a secondary ground cable (pink arrow):
View attachment 47013
These are often left disconnected after engine repairs (or missing). Most of our cars, this is a bare braded stainless-steel cable (like this one). Again, sometimes the cable needs to have ends cleaned (for rust or corrosion) or when reattached to a fresh engine, one might need to scrape off engine paint to get a good ground (just like above).
When Chrysler built these cars, they attached the negative battery cable onto engine, then painted the engine (so part of cable got painted) - just to make sure there was a good electrical connection.
View attachment 47014
White arrow.
BudW