Thanks, BMF. I'll keep the 13.7 v DC connection I have and either close up as is (with the red kit connectors) or rewire the connections to be smaller.
FYI, GE Parts sold me the GE heater kit (WR49X10173) after I gave them the model number PS123DGM. I've seen other non-appliantology posting showing the heater installed in a 4-wire harness refrig, but the wiring change from a 6-wire harness was never noted. Also, the kit says its for metal liners only, although again many are using on plastic liners and say the heat isn't enough to matter. (I'm not certain what this means since all the refrigs shown seem to have an external plastic liner in the water/ice dispenser slot.)
I'm seeing this 4-wire harness problem many places and no one (especially GE) is addressing it so far. GE is selling the part and my refrig has the slot for the heater, just not the 6-wire harness. I suspect the real problem is GE (which should have owned up to the design flaw long ago). GE discounted the part to $30 plus shipping, making it only a little more than the online price. It does come with a 1 yr GE warranty. GE should give it away (as some reported) and update the instructions.