I measured an extra panel resistor at 5 ohms just now. At 12v the 8 panel lights (4 center dash, 4 on tach/speedo) produce 17.6 W (8x2.2w), consuming 1.5A in the process on the bright setting. Assuming 5 Ohm for the resistor on dim, the lights produce 7.5 W at 1.5A. Generally this current will be lower since the nominal voltage is about 13.6v.
So a 5 Ohm, 7.5 watt resistor would be sufficient (I’d use a 10 watt for a margin of safety). Make sure to use a wire wound resistor, and be careful on the resistor body length.You can easily find the resistor at Mouser.com.
Purchase several maybe a 5 Ohm,. 7.5 Ohm, 10 Ohm and see which looks best on the dim setting (just temporarily connect this resistor in place of the panel resistor)
I went a different way and installed LED lights, using a different resistor value to get the brightness right. I used a 3W 40 ohm wire wound resistor (Ohmite brand) obtained from Mouser.com. This will not work for the incandescent lights.