If it's styrene to styrene, with styrene glue, you could try putting the piece in the freezer. The low temperature will make it easier to break the pieces apart along the bond.
If it's CA glue, a little acetone will soften and dissolve the glue, but use it sparingly to avoid damaging the plastic. Of course, with CA glue, a little stress at the join might be sufficient to break the glue.