I suspect your 1984 GTI will be very similar if not identical to my 1984 Storm.
The vacuum line on the back of the cluster runs to a connection next to the check valve right above the manifold, right behind the valve cover. Take a look at the engine bay photo on Richie's Scirocco website here:http://website.lineone.net/~dr.dub/mk2ext.htm
You can see the vacuum line running under the right hand silver clamp by the throttle body (the line is white there). It runs under the throttle cable, and connects into a black line which in turn plugs into the fitting next to the check valve. Also look at page 93 in the Haynes manual (figure 3.62). There is a second fitting next that check valve, but it is capped off with the green cap on my car. I accidentally removed the vacuum line to the cluster and the needle stopped working and the can ran a bit rough too.
With this vacuum line connected, the consumption needle should move, but only when you are in 5th gear. You won't see the needle in gears 1-4 at all. Instead in gears 1-4, you will get the amber "upshift" LED if you are at an RPM/throttle combination where you could be using a higher gear. For example, 2100rpm with light throttle.
If the upshift LED isn't working, there is a solenoid attached to the distributor. Make sure the vacuum line hasn't come undone there and that the electrical connection is ok.
Also, there is an electrical switch which plugs into the transmission which senses reverse (to illuminate the backup lights), and also 5th gear (for the MFA).
Hope this helps. If you need a better picture of the engine bay, PM me your email address.