IIRC the newer trucks - maybe 94 - 95 had the EST connector (Black/Tan?) under the dash near the PCM. The older trucks had it in the main loom near the fire wall close to the power distribution block. Here's the location on my 88.
Edit: You have to dig it out of the loom if nobody's ever done it before.
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Edit: You have to dig it out of the loom if nobody's ever done it before.