I think this might be a Tomb Kings only problem, but what happens when a character has an option to go on a chariot which has a 5+ armour save and is also wearing armour himself? Does this stack? Being mounted on a steed or monster definitely does, but there doesn't seem to be anything specific on chariots...
No it does not stack. Like a ridden monster, a chariot and its character rider can be attacked (and killed) separately. As such, they each have their own independent armour save. Of note though, the character's armour save is improved by one for being mounted. You'll find the relevant rules on page 105 of the BRB.
Nice! Wow. Just when I think I've really duplicated all of the parts of the BRB something like this happens. Literally never saw this bit somehow. Thanks.