  Consideration of both low- and high-resolution transmission spectroscopy is
key for obtaining a comprehensive picture of exoplanet atmospheres. In studies
of transmission spectra, the continuum information is well established with
low-resolution spectra, while the shapes of individual lines are best
constrained with high-resolution observations. In this work, we aim to merge
high- with low-resolution transmission spectroscopy. We present the analysis of
three primary transits of WASP-69b in the VIS channel of the CARMENES
instrument and perform a combined low- and high-resolution analysis using
additional data from HARPS-N, OSIRIS/GTC, and WFC3/HST already available in the
literature. During the first CARMENES observing night, we detected the planet
Na D$_{2}$ and D$_{1}$ lines at $\sim 7\sigma$ and $\sim 3\sigma$ significance
levels, respectively. We measured a D$_{2}$/D$_{1}$ intensity ratio of
2.5$\pm$0.7, which is in agreement with previous HARPS-N observations. Our
modelling of WFC3 and OSIRIS data suggests strong Rayleigh scattering, solar to
super-solar water abundance, and a highly muted Na feature in the atmosphere of
this planet, in agreement with previous investigations of this target. We use
the continuum information retrieved from the low-resolution spectroscopy as a
prior to break the degeneracy between the Na abundance, reference pressure, and
thermosphere temperature for the high-resolution spectroscopic analysis. We fit
the Na D$_{1}$ and D$_{2}$ lines individually and find that the posterior
distributions of the model parameters agree with each other within 1$\sigma$.
Our results suggest that local thermodynamic equilibrium processes can explain
the observed D$_{2}$/D$_{1}$ ratio because the presence of haze opacity mutes
the absorption features.
