Vacuum Switch ?? Mounted on right strut tower?

The upshift indicator circuit uses a vacuum switch (actually, two, but one is labeled "Upshift solenoid valve" in the Bentley). Try measuring continuity of the switch both at idle and with the engine stopped. There should be continuity in one case, not the other (don't remember which is which). Mine broke earlier this year and I replaced it with one from the junk yard.

The upshift indicator control unit takes a number of inputs, the following applies to A2, A1 should be similar:

1. Vacuum switch under the hood
2. Limit switches on the transmission
3. Vacuum switch in the instrument cluster.
4. Tachometer signal from the coil
5. Power and ground

They serve the following purposes:

1. Detects engine vacuum, if there is too little vacuum (too high absolute pressure) the upshift light will not come on, as this means the engine is under heavy load (you are accelerating)
2. Detects engagement of 5th and reverse gear. In those gears you can not possibly shift up, so upshift light is disabled.
3. This is not well explained anywhere, and is based mostly on my own observations. If you are not stepping on the gas at all, like when using the engine to brake going down a hill, there will be a large vacuum (low absolute pressure). However, you do not want to upshift in this case. I believe what is described as a "Vacuum Solenoid Valve" in Bentley is actually another vacuum switch used to detect large vacuum. If the vacuum is too high, the upshift indicator will be disabled.
4. Back to documented facts. If neither of the previous three items have disabled the upshift indicator light, it will turn on above a certain engine speed (RPM) based on the tachometer signal.

If you have the plumbing for the upshift indicator light but not the actual light, perhaps a previous owner has replaced the instrument cluster in the car?

Jim B.