OK, you need to test the resistance at the wiring harness. Two of the wires are for the lights. The other two you need to measure the resistance across them.
When you press buttons, the resistance should indicate:
Volume up - 1564 ohms
Volume down - 1912 ohms
Band - 3102 ohms
The service manual says in one place that you have to test across A & B, and in another it says A & C.
I would say try to find the two that will light the lamps with 12V, then you know it is the other two that you need to test.