There are many references for the representation theory (say over $\mathbf C$) of $SL_2(\mathbf{F}_q)$ and $GL_2(\mathbf{F}_q)$, for instance lecture 5 in Fulton--Harris "Representation theory" and section 4.1 in Bump's book"Automorphic forms and representations". Is there anywhere I can find a similarly explicit description of the representation theory of $SL_2$ and $GL_2$ over the ring $\mathbf Z/n$, where $n$ is not a prime? (By the Chinese remainder theorem, it suffices to consider the case when $n$ is a prime power.)

There are many references for the representation theory (say over $\mathbf C$) of $SL_2(\mathbf{F}_q)$ and $GL_2(\mathbf{F}_q)$, for instance lecture 5 in Fulton--Harris and Bump's book. Is there anywhere I can find a similarly explicit description of the representation theory of $SL_2$ and $GL_2$ over the ring $\mathbf Z/n$, where $n$ is not a prime? (By the Chinese remainder theorem, it suffices to consider the case when $n$ is a prime power.)