How do you get NPCs to wear armour in the construction set after the morrowind code patch is installed?
I only noticed this problem when I was trying to make my first real mod and needed to see what the base (unmodded) armour looked on a NPC (so I can choose what armour that i was going to use for the recolouring), only to realise that they wouldn't wear any of it. If anyone can help me then thanks in advance.

An NPC will only wear armour from the class that they are skilled in (Light, Med or Heavy.) If their Unarmored skill is very high, then they won't wear any.

This doesn't have anything to do with the code patch.

So if I just put their armour skill really high, they should start wearing it?
The reason I thought the code patch was causing it because it stops the merchants from wearing any armour they obtained so I thought it applies to construction set NPCs.

You can do that, but it's not needed in most cases, and will make for an unbalanced fight if the NPC is meant to be killed.
Armour proficiency is based on which Class the NPC is, so either change the Class or use armour that is suitable to that Class.