Your asking for trouble doing this.  The connectors are probably old
enough that if you start unplugging things that plastic will start snapping
then
you will never get the new harness together tightly.  If you absolutely
must do this, then the trick is to take a shop towel and wind it around
the connector, pour very hot to boiling water on the towel, then quickly
while
the plastic is still warm and pliable, and using neophrene gloves, to
disconnect the fitting.  At the wrecking yard, you merely cut the
wires upstream of the connectors and take the assembly home and
separate it with the boiling water trick.  I would suggest starting
with the wrecking yard harness first to see if you can get it apart without
snapping
the plastic.

If you don't know how to properly splice wires then take it to an auto
electrician.  Your talking 1/2 hour of work with a solding iron and
heatshrink tube to do a proper splice.  If there's still a hitch on there
they
can probably even do a proper splice of a trailer harness socket on to
the vehicle so you could use the hitch.

Tap connectors serve but 1 function, they are cheap and quick and only
require a pair of pliers to put on.  So, the untrained monkeys at U-Haul
can quickly get the harness connected for the U-Haul trailer that the
customer has rented.  The untrained monkeys don't give a crap that
6 months later that the mass of electrical tape they wound around the
tap connector to keep the water out has unwound itself and left a sticky
mess behind, and water has corroded the harness into a mess.  That's
someone else's problem.

Ted