Certification for automotive accessories?
There is no specific certification for the whole assembly. This is because the requirements vary wildly based on the vehicle manufacturer. There is a short list of standards that you will have to comply with- here are big three in my mind:
AEC Q100 / AEC Q200 - component specs. This is easy- only use automotive components, unless there isn't something available. If you need to use non-automotive parts, you need to perform this testing or justify it to the OEM.

ISO26262- functional safety spec. The key here is ASIL - Automotive Safety Integrity Level. You can think of this as being based on how likely a failure is and how bad the results of a failure are (in reality it's a bit more complex). You do your own initial assessment, and then the OEM will look at your assessment and see if they agree. They are often very conservative - keep in mind that any repairs within the warrantee period of the car look bad for them.

CISPR 25 - Electromagnetic Interference for vehicles. Similar to EMI requirements for any industry, but different frequency to amplitude curves. The AM band is usually the most problematic in my experience.

Some OEM's will have their own requirements in addition to the basics. This is where some of the more tricky specs will come into play. Some OEMs will require Design Failure Mode Effects Analysis (DFMEA), Production Failure Mode Effects Analysis (PFMEA), Advanced Product Quality Planning (APQP), or other standards. Every OEM also has their own specs for how electronics need to be designed and manufactured.

By far the most common approach for a startup or small company trying to enter the automotive space is to engage with an existing tier-1 manufacturer, so you can leverage their expertise in a development agreement to get your product to comply with the myriad specs, and to access their ability to sell into the automotive supply chain. You should also select a contract manufacturer who has automotive experience so you can be sure they'll be able to comply with the APQP / PPAP requirements during production.

Best of luck- please follow up with any additional questions you may have.