Hyundai and Kia added official GrapheneOS support to their apps months before Volkswagen banned GrapheneOS:

https://discuss.grapheneos.org/d/31697-myhyundai-app-has-official-grapheneos-support

Pressure from Volkswagen customers on them can achieve the same thing. There's no legitimate reason to ban GrapheneOS so they'll undo it with pressure.

myHyundai app has official GrapheneOS support - GrapheneOS Discussion Forum

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GrapheneOS Discussion Forum
Leave a 1 star review for Volkswagen's apps on the Play Store asking them to stop banning GrapheneOS. Explain it's a far more secure operating system and fully possible for them to verify the hardware, OS and their app on it if they insist on doing it. It's far more secure than anything they allow.
Google has misled companies about what the Play Integrity API provides. It doesn't genuinely enforce having a secure device or legitimate app, it only pretends to. It leaves huge security holes open. It enforces Google's business interests and bans having a reasonably secure device with GrapheneOS.

Most companies are unlikely to stop using the Play Integrity API but most are willing to start permitting GrapheneOS via hardware attestation with enough pressure.

In addition to every user of their app on GrapheneOS leaving a 1 star review on the Play Store, multiple other steps can be taken too.

Every GrapheneOS user with one of their cars using the app should file a customer support request. Keep answering them and countering the template responses. Escalate the request higher up. Tell them you want money back for the vehicle due to reduced functionality after the fact and insist on it.
They can trivially stop enforcing the anti-security and anti-competitive Play Integrity API or easily add hardware-based verification of GrapheneOS. Link to https://grapheneos.org/articles/attestation-compatibility-guide in the customer support request, but don't add any links to Play Store reviews to avoid filtering.
GrapheneOS attestation compatibility guide

Guide on using remote attestation in a way that's compatible with GrapheneOS.

GrapheneOS
A bunch of apps have added explicit support for GrapheneOS due to pressure from our users. Our userbase is rapidly growing and we'll gain the ability to apply massive pressure to companies doing this. We plan to ship a feature for our Info app for people to opt-in to getting asked for their help.
@GrapheneOS i dont know If the Fairphone 3 is supported, but i would love to Switch to graphene, after the Support ran out

@BigDvsRL GrapheneOS is not an OS to use on end-of-life devices. It's a production quality OS with far stronger privacy and security than standard AOSP and especially regular Android devices.

Fairphone 3 is an insecure device on an insecure end-of-life Linux kernel branch for years already. Fairphone misses many standard privacy or security patches. The ones they provide come many months late. It was also missing important hardware-based security features at launch and is missing far more now.

@GrapheneOS yeah, noticed that after googling 😓