ECDSA: What is the purpose of adding a hash of the used certificate to the signed data?
It is probably a countermeasure to multi-user attacks.

In the classic (single-user) attack scenario the attacker wins if she manages to create a valid signature under a given public key, without knowing the private key. (note that a it is not request to obtain the private key, but just one valid signature)

In the multi-user scenario, instead, there are multiple public/private key pairs. And the attacker wins if she's able to create one valid signature under just one of the given public keys (the attacker can choice the target one).

The multi-user attack is believed to be easier than the single-user attack.
The high level idea is that the attacker's computation can be reused against any of the allowed public key, thus increasing its efficiency.

A common countermeasure, introduced in ed25519, is to create a stronger bond between the signer and the signature by including in the hash computation the signer's public key. 
What you describe in the car2car scenario looks similar to that countermeasure as the forger needs to create hashes of the messages which include the signer's identity. This most likely remove the multi-user advantage compared to single-user.

Bernstein showed that adding the signer's key in the ed25519 hash computation protected against multi-user attacks.
Klitz et al. later showed a reduction from the multi-user to the single-user. Bernstein, however, disagrees with the paper conclusions. 

To summarize, IMHO, adding the signer to the hash is very efficient and seems to provide a generic solution to concerns about multi-user attacks, while the reduction between multi-user and single-user for signatures is still unclear.