This disconnecting of dash cams and then trying to hide behind Gdpr is total and utter bo44ocks. It's only done so you can't see them ragging the car about! Absolutely boils my pi44.
They tried that on me. I asked their head of service (who was involved at this point) to explain it to me precisely and which articles applied in this context. I didn't mention that I deal with it all the time on behalf of clients and have in house counsel whose job is solely devoted to data security and compliance. The answer I was given was such nonsense.
If a dealer (or more likely, the parent group) is FCA registered, they have to appoint a data protection officer. They take it seriously because they understand the risk and the fines. Just take a look at
GDPR Enforcement Tracker and filter by UK. The fines are percentage based on turnover in a fiscal year. Cabinet office were fined 500k recently. But if you're unhappy with how they've managed data, a letter to their DPO produces inspired results thanks to the above.