I try to let PI work with RTW 1.5 (no BI 1.6, no RTW Gold).
You need to create an empty BI file within your RTW file, to put the PI file within the BI file and to change the two data files below.
When I want to continue the campaign (there is an auto save file), I am told PI/data/PI/models_strat/PI_diplomat.cas doesn't exist.
PI\data\PI\models_strat\PI_diplomat.cas does, but has back slashes as you can see.
I don't manage to play the campaign through the main campaign (or the provincial campaign) or to play a custom battle. Help is welcome.
EDIT: The trick that worked for RNJ doesn't seem to work for PI.