Could be anything really, depending whether something was changed or work done to the bus that caused the fault, or whether a part of the circuit (relay, connection) failed all by itself.
It's well worth downloading the wiring diagram for your year bus and tracing it through.
You could measure the voltage between a good ground and the connection of the brown/white wire to the relay.
You would expect it to be 12V until you pull the stalk switch, when it should go to 0V (ish).
If that connection doesn't go below 3V, especially if it is different with ignition on, that might not be low enough to operate the relay.
Pushing a spare bit of wire into that brown/white connection and connecting it to ground should operate the relay and that should tell if the fault is in the relay itself or in the stalk circuit.
It is also possible that there's a poor connection in the circuit to the + side of the relay coil (loose or corroded fuse?), such that when you turn on the ignition there is a big enough volt drop in the supply to the relay that it can't operate.