Coherent backscattering (CBS) of light in a strong localization regime is studied with absorption/ amplification included. The CBS cones arising from strongly localized states inside the complete gaps of disordered photonic crystals have been observed by numerical simulations in two dimensions. The structure of the CBS cones can be described well by a theory that incorporates mean free paths, localization length, amplification/absorption length, and single-particle scattering anisotropy. The excellent agreement between theory and simulations without any adjustable parameters makes it ossible to determine the localization length from a measured CBS cone, even in an absorbing medium.