I ran mine from a pre-existing hole in the chassis to the bell housing. If there isn’t a convenient hole that is unused, then take a bolt out of an engine mounting and use that point. Just ensure that the surfaces are clean so that you get a good earth connection. The point above re using a jump lead to prove it is the earth is a good one and could save you a lot of time.
If it ends up being the solenoid, then you may need a new starter motor - if I remember correctly, the solenoid is an integral part of the starter motor. My cautionary note is that it is nearly 8 years since I sold my 1985 4/4!
