Do I have to have the parts registered with the TÜV (German Technical Inspection Agency)?
Yes, any structural modification must be
approved by a test center. Otherwise your operating license will expire. We
already offer many items with a TÜV certificate, so you can drive in traffic
with confidence. It's best to talk to the TÜV inspector about your project
beforehand and have it approved.
https://dev2.bartek.426.agency/help-center/products-stock/do-i-have-to-have-the-parts-registered-with-the-tuev-german-technical-inspection-agency