No need to buy a new harness. That probably costs big $$.
Just unwrap the tape from the bindle of wires that are at the Trunk lid / body intersection. Spread all those wires apart after you remove the tape and look at them closely. It is a tight fit but it can be done.
See if there is a broken or almost broken wire. If no broken wire then the short is probably not the fault of this bundle of wires.
On mine one wire and only one wire was worn almost through (not being worn all the way allowed it to work sometimes).
Then take the worn wire and snip it in half at the wear point. Then splice it. Use butt barrel crimp connectors. (see this address for a good tutorial on splicing wires with butt barrels:
http://www.mmxpress.com/technical/connections.htm .
After you make the splice, check to see if everything works. If all OK re-wrap the wire bundle and be happy.
I had to look really close to see the wire that was faulty. The insulation was still on it but the wires inside the insulation had begun to frey apart.m I wiggled each wire and had my wife watch the tail lights as I did this inside the trunk. When I wiggled the freyed wire the lights went off and on.
Good luck
