BTW, I wonder who your mechanic is???
I am in the US and the engine for 98 V70 is 2.3L; yours is 2L so I assume you are Europe.

Troubleshooting ANY electrical systems requires:
- knowledge of the ETM (Electrical Troubleshooting Manual)
- patience
- persistence
- experience

Obviously, parts were thrown into the car unnecessarily in your case.... Hope it did not hurt your pocket!

I just checked the ETM for you:

The Control Module relies on sensors and controls the relay through pin #17.
Pin 17 is Y-W and leads to 1B on relay.

(Relay's 2B and 1A are controlled by fusible link from starter motor, i.e., when starter is enggaed, fan is disengaged to save battery drain to give starter motor the most current as possible).

Relay then sends out 2 ports, 1C....Gr (controlled by Module), and 2C...Red (controlled by starter motor).

**** The fact that you have new ECU, relay, temp sensor, and fan works with external source tells me something is wrong with the wiring because the components are good.

So:
-disconnect wiring to relay
-apply 12 v directly to green wire of fan:
a. if fan runs, problem is upstream of relay.
b. if fan does not run, then ground is no good (because your fan works with external source of power)
I SUSPECT bad ground for fan. If this is the case, remove Brown wire, clean with sand paper and reconnect, apply a bit of grease on the outside to prevent corrosion (the brown wire sits in front of the car , so prone to road salt corrosion).

Remember your fan runs only under certain condition: temp hits certain deg C.
Turing the AC on does not mean fan will run.

SO maybe there is nothing wrong to start with. Fan only runs when it is HOT.

Let me know what you find.

HTH.

cn
98 V70 75K miles
Omaha NE
cnn90@netzero.net