I think the IPC should light up when you press the TEST button regardless of what the BCM is doing. Seems to me like a test button that is part of the IPC should override all external inputs assuming the IPC has power going to it. I don't have a way of doing any testing anymore or I would unplug the BCM and see if the test button still lights up the fluorescent display to prove the IPC is most likely the problem.
Here is the bottom line for me... Don't just assume that Cluster Fix knows what they are talking about. If the IPC doesn't pass the test below, taken from the '89 FSM, buy or borrow an IPC that you know is in good working order from someone you can trust and see if that fixes your problem. Never hurts to have a spare IPC anyway.