The automotive industry is shifting from hardware-centric development toward software-defined vehicles (SDVs), where software drives functionality, value creation, and competitive differentiation. Growing software complexity renders firm-centric and proprietary software development models insufficient, prompting a shift toward ecosystem collaboration among OEMs, suppliers, and software firms. Yet, how these SDV ecosystems emerge and operate in response to software-driven development remains insufficiently understood. This study enhances our understanding of SDV ecosystems, outlines their collaborative structures, identifies stakeholders, their roles and authority, and highlights associated challenges and opportunities. This study identifies six levels of collaboration involving twelve stakeholder groups shaping SDV ecosystem transformation. These collaborations are influenced by five dimensions of authority. SDV ecosystems face six core software development challenges alongside six organisational, six industry and market, and four regulatory, legal, and ethical challenges. The literature also highlights five key software development opportunities complemented by six organisational, four industry and market, and two public value and ethical opportunities. SDV ecosystem research is primarily technical, concentrating on architectures and standardisation, while lacking studies on governance and collaborative software business models that reflect regional characteristics and power dynamics. We reposition SDVs as multi-level socio-technical ecosystems where software functions as the core structuring principle but does not alone determine ecosystem success. We develop a multi-level SDV ecosystem model, integrating stakeholders, collaborative structures, and governance across ecosystem levels, and outline directions for future research and practice.