If you have an 05-09 Mustang, open the deck lid, pull off the plastic cover that is attached and disconnect the trunk release wire harness. Follow the harness to the union plug, you will see the white wire/pink stripe. This wire follows under the passenger side door sill, up to the junction box. You can tap into this wire anywhere along it's route to the junction box. I believe it is a 14g wire. Once it is grounded, the trunk release will trigger (after you re-connect the harness!)
It really is a simple install. You may just be using the wrong trigger wire. and it has to be in the harness that comes from the right rear of the car, before it enters the junction box. Good luck!