We're not sure why a trunk release wasn't included on the 2005-09 Mustang, but we do know that we needed to fix it! Available now is the SHR trunk release kit, which includes a factory switch (not some cheap imported momentary button that doesn't fit correctly), soldered connections, and heat shrunk protection of terminal ends.
This kit installs in minutes, requiring one hole to be drilled in the glove box interior for a clean, OE-type install. No wire cutting is required, and full instructions are included. All wiring is done in the front passenger area of the car, no wiring to the back is required.