  We use soliton methods in order to investigate the interior electrovacuum
region of axisymmetric and stationary, electrically charged black holes with
arbitrary surrounding matter in Einstein-Maxwell theory. These methods can be
applied since the Einstein-Maxwell vacuum equations permit the formulation in
terms of the integrability condition of an associated linear matrix problem. We
find that there always exists a regular inner Cauchy horizon inside the black
hole, provided the angular momentum $J$ and charge $Q$ of the black hole do not
vanish simultaneously. Moreover, the soliton methods provide us with an
explicit relation for the metric on the inner Cauchy horizon in terms of that
on the event horizon. In addition, our analysis reveals the remarkable
universal relation $(8\pi J)^2+(4\pi Q^2)^2=A^+ A^-$, where $A^+$ and $A^-$
denote the reas of event and inner Cauchy horizon respectively.
