As to the factory harness: it seems to be buried in the WIS CD. TSB
300-9017, Jan 1996, describes the built-in wiring:
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In sum, the 5 pins are:
1 rd Power from fuse 23
2 ye/gy Power from Fuse 17 (keyswitched power)
3 gy/wh from rheostat 16, the panel illumination dimmer
4 bk Ground
5 gy - mute
Therefore, it isn't terribly useful. But perhaps necessary, given that most hands-free kits only connect to 4 speakers, and thus we need some way to mute the door speakers.
Later models have a slightly more advanced setup which can be modified nicely:
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(has a nice pinout of the later version of the telephone harness)Given that later models mute is active-low, my guess is that older models are also.