Pink box

I’ve been working on some of my own modding recently and I’ve managed to figure out how to add a unit from one civ to another civ. The unit functions as it should, however generally the unit shows up in the game as the pink box saying “this unit is not cashed” or something like that.

As examples: I added the Desert Raider to the macedonias and got the pink box.

But I added the Desert Raider to the Byzantines to be built at the Stable and that unit works and looks fine.
I am guessing this is because the Desert Raider is already availible to the Byzantines through the Market/Mercenary House and as such the units graphic model is already in the Byzantine civ file.

Also I added the Tower Elephant to the Tughlaq and the Elephant shows fine, but it has 2 pink boxes on its back. Im guessing this is because the graphic model for the Elephant is in the Tughlaq file but not the graphic model for the archers/crossbowmen on its back.

So how do you add a graphic model to a civilization so that the pink box wont show up?

Thanks in advance for any help!