the ground is located right below the latch pin under the the plastic trim, at least in sedans and hatches.
go to post 14 , pic 3, in this thread. also notice the main wiring harness connectors are located on the drivers side in the rear corner. (there are also some pics not showing, but the links can be clicked)
ZX3 Trailer Lights Wiring Tips
Take pics of what your doing, do a write up, and post them up.