Accurate prediction of water propagation in vehicle interiors is increasingly important as modern cabins integrate numerous moisture‑sensitive electronic components. Typical spill and rain‑ingress events generate complex, transient free‑surface flow that is often routed through narrow structural gaps. High‑fidelity meshfree CFD approaches based on the Generalized Finite Difference Method (GFDM) can resolve such behaviour, but computational cost rises sharply when fine point resolutions are required for small gaps embedded in complex geometries. To mitigate this limitation, a Line Boundary Condition (LBC) is introduced as an efficient reduced‑order representation of confined gap flow. The LBC models narrow gaps using geometric line segments equipped with artificial sink‑source terms that approximate local mass transfer through gap openings while guaranteeing global mass conservation across interior interfaces. This enables coarse and spatially uniform point distributions while maintaining physically consistent mass exchange between adjacent compartments. The LBC is integrated into a fully three‑dimensional GFDM solver and evaluated on realistic automotive interior assemblies containing multiple interacting gaps, heterogeneous surface materials, and non‑uniform flow paths relevant for spill scenarios. Since micro‑scale gap details are not explicitly resolved, validation focuses on reproducing plausible accumulated‑mass ranges, wetting‑sequence likelihoods, and dominant transport pathways rather than exact quantitative mass distributions. Comparisons against physical tests and high‑resolution simulations show that the LBC captures experimentally observed mass‑flow trends, correctly ranks the dominant flow routes, and reproduces major discharge tendencies over time. Across all configurations, it achieves more than an order‑of‑magnitude reduction in computational effort while preserving predictive value for water‑management engineering tasks. The method therefore provides a practical extension for large‑scale interior moisture‑risk assessment. Ongoing development targets automatic gap extraction from CAD and calibration strategies for non‑uniform, curved, or partially obstructed gaps.