well I guess its more a mod than a hack...
From a 996 owner's blog page:
Kevin's 4S Page
"Shift Light Installed.
I really got used to having a shift light on my old track car and really missed it. Especially since this car revs so fast and 1st and 2nd gears go by so fast. I really wanted a sequential multi light shift light as opposed to the single light one I had before (MSD). This way I have a good idea of where I am in the RPM range and can better anticipate when to shift. After searching around the various enthusiast forums I found one by the Austrailian company Ecliptech. After ordering I got it in four days. Ain't that amazing! And the shipping was reasonable too. I read several threads on Renntech about where to find the appropriate wires, and find out how to take the instrument cluster apart. It took me an afternoon as I never had taked apart the dash on a Porsche before, and I really took my time. Everything is nicely engineered and easily went back together, unlike other cars I've worked on where things never seem to want to go back together easily and look the same as when you first took it apart.
I should have taken more photos, but on the left you can see the aluminum 'T' I made. You can't see it well in the second photo, but the top half of the 'T' is bent down thus allowing the shift light to have an attachment surface. The photo on the right shows the existing gap in the gauge trim that has been slightly enlarged so that the aluminum 'T' I made would have clearance to pass through. I did not glue the 'T' to anything as the tight fit between the gauge trim and the top dash piece. I did stick the 'T' under the existing padding. I forgot to take another photo to show this, but about an inch over from the existing slot that is shown in the photo on the right, I cut a similar slot, but smaller, for the shift light wiring.
Wiring: 3 wires on the shift light, black, black/red, black/blue.
Black => ground => brown, there are four easily accessable grounds right behind the instrument cluster.
Black/Red => ignition on, switched 12V => green/black, I used the unused phone plug located under the radio in the center console. Take off the carpeted panels (down by your feet) and you'll find a plug that is not connected to anything. Tap into the green/black wire.
I have it set so that the first light comes on at 5,000rpm. At 6,500rpm, after the lights have all come on, they all flash. I figure this also gives me enough time to do something before hitting the rev limit. "
Black/Blue => tachometer => violet/green, this wire can be found on the OBDII plug located next to the fuse pannel"