First things first, the factory service manual can be found in the resources section on this forum. Wiring diagrams are in section 8W, but can be somewhat complicated to read because they're laid out by functional area, and you have to jump across many pages to find your way. But because I'm recently laid off and enjoy the wiring, I'll take you through what I did you answer your question. Teach a man to fish, as it were.
I happened to already know that the cluster is connected through 2 connectors, C1 & C2. The pinouts are found on pages 8W-80-22, 23. Pin 9 on C1 and pin 6 on C2 are the ground wires. They're both BK/LG (black with a light green trace), 18ga wires which are part of circuit Z2.
Section 8W-15 is the ground distribution chapter, and page 8W-15-1 has the index. Instrument cluster is on pages 8W-15-5 (assuming you have a LHD), and it shows that both wires are grounded at G201. You can see these are the same wires because they're the same color, gauge, part of Z2, and also correspond to the same 6 & 9 pins. If I knew/remembered this section existed I would've gone directly here. I find that it's usually easiest to start at the connector, because I can start to build a full circuit schematic in my head if I start there.
Section 8W-90 has the connector & ground locations, and you can see that G201 is found "Left rear of instrument panel" in figure 17. From the looks of it, it appears to be on the back side of the dash, behind/below the driver's side speaker. "ish." So to answer your question, that's where the instrument panel grounds are found.
* I did these lookups on a different computer. I may come back and add screen caps because I'm bored.