There is a 2 pin terminal on the rear of the cluster - behind the oil gauge. One of these is the tacho feed signal, and the other is the oiul pressure signal.
All VH SL/Es & all V8 models had a tacho dash fitted standard, and the wiring for it has 2 wires in a plug that goes onto this - one is blue (oil), the other brown (tach).
VH SL/X models had the oil pressure, but no tach, so they had only 1 wire there (the blue one).
VH SL models had nothing on there.
Basically what you need to do is connect a wire into the empty bullet terminal on the - side of the coil, and run it into the car & connect it to that terminal on the back corner of the cluster for the tach & it will work.
If you want to install it like the factory, then look at the multi-pin engine connector on the firewall (has green, blue & pink wires, amongst others in it) and you 'll see on the car side is a brown wire, but an empty terminal on the engine side.
When the dealer (or holden) fitted a tacho, they plugged into this wire & then found where it was taped back to the harness under the dash, plugged in the short patch loom & plugged in the dash. That way you don't have to poke a new hole in a firewall grommet.
The other concern is the oil pressure gauge. SL had only a light, and the sender is completely different in operation. Does your oil pressure gauge fire off the end of the scale when you turn on the ignition, and then drop below zero once the car is running? If so, replace the sender switch with the variable sender unit that suits the gauge.
I'm not sure if lower spec models get it or not, as I've played mostly with factory SL/Es in the VH, but the oil pressure wire that goes to the light may need to be diverted to the gauge terminal on the rear of the cluster (this is already incorporated in the SL/E dash harness, but I'm not 100% sure about SL & SL/X dash harnesses.
The SL/E does run a completely different IP harness as it has an external amp for the stereo, additional wiring for the 2 dash speakers, and a 9 pin plug for the trip computer (whether fitted or not). Lower spec models don't get any of that, apart from the trip comp plug - but it's only 6 pins & needs a patch harness.
That's the main way I catch out dodgy people flogging fake SL/Es - even if a numbers jpb has been done on the build plate, most don't bother to change the I/panel loom.
