After Continental AG acquired Siemens VDO, they released a legacy technical portal. Search for "Continental Aftermarket RD3 manual." You may need to register as a professional.
If you have a specific symptom or repair need (e.g., "no display but audio works"), let me know and I’ll give a targeted step-by-step guide.
Avoid "cheap" manual resellers on eBay. They will sell you the 2-page user manual, not the 98-page component-level service binder.