Ecotec Build

The speedo is hooked to the transmission via a speed sensor.  If you're using a stock tranny then no problem.  Otherwise, you might need to get a converter to massage the existing signal of whatever tranny you have.  Not hard to do.

The Tach hooks to the coil circuit typically.  There is an easy way to make the Tach adjustable.  You simply need a multi turn pot and some soldering skills.

The Dawg

Should - but a dakota digital box will also convert the signal, and let you keep your stock gauges.

The VSS generates an A/C signal doesn't it? If so, you need both wires to see the whole wave.  I could be wrong.  ;)

If it's a square wave, and one side is referenced to ground, then...  yeah...  one wire of the VSS could be grounded, and the other fed into the Dakota box (don't confuse it with your Lakota box though, or an elderly native gentleman will come and talk to you about your muscle aches).

Look on the GM diagrams, and see if they're marked VSS hi/low.

Oh my god... Craker, that was the worst joke in the history of... Ever.

I think, in the end, John's right. Typically (Read: Stock) both halves of the signal are used, as the VSS is a AC voltage generator...

All of them are AC as they come out. If it has a module on it, that'll convert the signal to a DC frequency signal, but every one of them that I've seen are two wires in, one wire out.

Ok!  Then there's your answer.  Sine wave output.
Gets converted to the aforementioned square wave up stream....

You could TRY grounding the purple, and see if you get a sine wave (a/c) between yellow and ground.  If so, then stuff the yellow into the dakota box (I'd guess they're referencing to ground for it's input).
 

Here's an audio clip of the Ecotec.

2011-05-01_080758_Ecotec_Sound.rar

Ok,ok, I'll do a proper recording and give it a litle juice.
Just for you.