Here is the email directly from JHP. While it does not specifically answer the question of the wiring dates, there is a solution. They guages are most indeed on my Christmas list as well.
Thank you for your enquiry about the JHP GTO sports gauge kit to suit your
Depending on the build month of your GTO the install is easy.
Pre March '06 its plug & play $460 shipped with grey dials / psi
Post February '06 will need wiring harness $540 with grey dials and you
will need the patch
harness wired into the dash harness.
The JHP 05/06 GTO Sports Binnacle Kit is the only factory style GTO Sports
Binnacle Kit on the market which comes with an easy to install plug and play
system. We include a step by step, pictured instruction guide which will
allow anyone to install the JHP 05 GTO Sports Binnacle Kit easily.
The JHP 05 GTO gauge pod has a LHD bias towards the driver which allows for
easy monitoring of the gauges. The genuine G.M. double humped design
provides protection from sun glare. Therefore, your gauges will be visible
and readable during the whole day.
The genuine factory gauges are color matched, and provide oil pressure and
voltage information. As they are genuine G.M. componentry they will also
illuminate in complete synchronization with your factory dash.
The JHP 05 GTO Sports Binnacle Kit includes factory gauges available in the
Red, Yellow, Impulse Blue, Barbados Blue, Grey, Purple, and fusion / brazen.