Yes it's for the PRND321 lights on the dash cluster, on VTII and VX Berlina/Calais and WH Statesman. All VT/VX auto models including VT series 1 - both V6 Ecotec and Supercharged engines, and the VT series 2 & onwards 5.7L V8's - have this socket on their transmissions. It was only on the VT series 1 that the PRND321 lights weren't connected up (i.e. a "plain" dash cluster fitted and plastic blanking cap on transmission socket/no wiring etc). This was because the old 5L V8 auto transmission didn't have such a socket so presumably Holden left this feature off the V6's so all VT series 1 models had the same features.
It's an easy mod to do and no you don't need Tech2. I hooked mine up on my VT series 1 V6 Supercharged back in 2007. Other than fit a cluster with PRND321 lights (obviously!) all i had to do is fit 4 wires between that auto transmission socket and the PCM (using pins off a damaged harness). You put the wiring through the firewall (carefully drill a small hole through then fit a rubber grommet in firewall, on passenger side above footwell behind glovebox etc). The auto transmission plug is hard to find at wreckers because most engine harnesses are sold with the engine (see 2nd pic for what the plug looks like). It's made even harder because you need to find a Berlina/Calais/Statesman harness which are rarer & all the lower model harnesses are missing the transmission plug. But the PCM plug is easier as obviously all models have 'em - you just need to carefully lever out four pins out of the wrecked/damaged harness to re-use on your existing PCM harness. It's fiddly but not hard. Then you just slot these four pins into the corresponding pin cavities on your PCM plugs with a small "click" (pins E6, E7, E8 and F15 - see wiring diagram).
Anyway i ended up finding an intact transmission plug on a wreck where there'd been an engine fire (so the engine and transmission were still in the car etc). I already had a spare old damaged VT Exec engine harness with intact PCM plugs.
You could of course just swap the whole engine harness with another "PRND321" one, but there was no way i was going to that trouble (and significant cost!) for only four wires.
For anyone interested in doing this mod, here are some wiring diagrams/pics that will help...
(the last three wiring diagram pics may be too blurred to read due to forum file size limits - if anyone wants the full version just PM me your email address.)
