If the engine crank when the positive cable is jiggled, it could be the BST connector. Have the X5 been in an accident?

You can use an ohm meter to see if there is continuity between the connector and along any length of the positive cable (you may have to use a sharp pick to prick a hole into the insulation and check the continuity).

Here's a video on just replacing the connector and not the entire positive cable.

https://www.youtube.com/watch?v=kXk4CqD8q2A

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