There should be a test tachometer connector in the engine bay on your car (5th gen, same as mine) located right by the brake and clutch fluid reservoirs.

I believe it's on the cars that didn't come equipped with tachs also, so mechanics (or us) can see the engine speed for setting the timing, etc.

I've never actually used it but I'd think you could hook up your gauge there and if you do it correctly and it still doesn't work, you'll know it's a faulty gauge.

Hope that helps