05 passat tdi starter wiring have hot wire to starter , trigger wire , but there is another wire maybe 8gauge or so that i cannot prove if its a hot wire or needs to be grounded ,

tdi engine was removed about 1 year ago and a used one was going to be
install but things came up and now i'm fuzzy on the function of this 3rd wire
with a regular terminal that would go over the stud where the battery wire is
on the solenoid but i feel this could be a heavy ground wire , testing with
ohm meter and test light did not prove anything .  i' m a ford master tech
and v/w is out of my comfort zone ,  thank you for any help