  The connection formula for the Jackson integral of type $BC_n$ is obtained in
the form of a Sears--Slater type expansion of a bilateral multiple basic
hypergeometric series as a linear combination of several specific bilateral
multiple series. The coefficients of this expansion are expressed by certain
elliptic Lagrange interpolation functions. Analyzing basic properties of the
elliptic Lagrange interpolation functions, an explicit determinant formula is
provided for a fundamental solution matrix of the associated system of
$q$-difference equations.
