It won't charge if the dash circuit is in tact and working. When you are in Key on/Engine off, does the battery light illuminate in the upper left pod of the IP? I think that there is a resistor that is in parallel with the light, so there might be an issue there.
I drove my 89 without the IP for a year or so, I soldered a resistor in place of the IP so that it would charge (actually I think it was a light bulb and resistor, I can't find it now).
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