It is well known that a random walk in random environment on a strip in the environment viewed from the particlesetting is a Markov chain on the set of environments. In this context, the fundamental question is that of the existence of the density of the invariant measure of this Markov chain with respect to the measure on the set of environments. In the case of the walk on a strip, it turns out to be possible to describe all positive sub-exponentially growing solutions of the corresponding invariant measure equation in the deterministic setting and then derive the necessary and sufficient conditions for the existence of the density when the environment is stationary and ergodic. This is a joint work with D. Dolgopyat.