[PLP]

And post for myself or others to know how to test radiators without PIWIS, Durametric or other expensive tools.

Turns out each fan and each speed has its own relay - yup, a total of 4 relays (53 model).
All of them are next to the fuse box in the foot well.

So if one of the speeds does not work - check the relay.
A 12 V battery charger will work - connect 85 and 86. Then test for resistance across 30 and 87.

Now, with all relays removed, test for power in pin 30.
Use a 12 or 14 AWG wire, strip 20 mm (about 1") of insulation. Using a hammer, smash the copper wire a bit to make it flat blade.
Now you can insert it to the socket.
Connect 30 to 87 (watch the relay orientation).
First insert to 87 and then to 30 (this way you do not have 12 V power in exposed wire).

Each speed will activate.
I do not recall the order, but I believe it was High Left, Low Left, High Right, Low Right - starting from the left to the end of the tray. Relays are 19, 20, 21, 22.

In my case - all relays work OK, low speed is gone on Right, while Left side is grossly imbalanced. Appears to be aftermarket fan which I read are known to be imbalanced.

It may help to remove the 4 screws around the fuse panel to drop the foot well side kick panel.