I've only done this cheat on The Ark so bear with me and yourself if this doesn't work!

So, on the rally point where you encounter the scarab tank with the Scorpion task force, kill all Covenant resistance and your UNSC colleagues.

Get aboard the scarab and kill all the Covenant crewmen, then get to the very front of the scarab just on top of the main anti-air cannon and you are driving the scarab, to turn, just face a different direction but DON'T move!

If this doesn't work then you haven't killed all the Covenant resistance.

If you have, then kill all the marines or you won't have the scarabs attention!