There are presently no brigades in the tech tree that use armored cars. What this means is that you can build them but then they can't ever be deployed in combat. They used to have their own brigade under armored brigades and an associated recon company (light armored recon) but those are both gone after BBA.
Edit: Some countries still start with armored car brigades in preexisting divisions, but you can never expand them by adding more armored car brigades in the division designer.