The circuit is indeed complex as you have a fan control module/ECU interface.

Even so, I think in this case it would be easier, and maybe even less expensive to fix the problem instead of "rigging" a bypass.

There are 2 likely causes and neither is an expensive repair.

1) The thermo switch on the bottom of the radiator is faulty

2) The coolant fan control module/relay has fried.

To see if the problem is the thermo switch on the radiator, disconnect its connector.

Get yourself a little piece of wire to use as a jumper between the terminals on the connector.

Your connector should look like one of these:

So if you have a multimeter, great. If you only have a test light that will work.

Turn the key to "ON".

You should have battery voltage (around 12 volts) at terminal #2 of the wire connector.

Now, use that little piece of wire to jump terminals 1 and 2.

The cooling fans should run at low speed.

Now, jump terminals 2 and 3.

The cooling fans should run at high speed.

If they do then replace the thermo switch on the radiator.

If you don't have 12 volts at #2, check your fuses.

So you have 1 fuse labeled "Radiator Fan Control unit" in your passenger compartment fuse box.

Make sure that's OK.

Then check the big fuses in the engine compartment for the coolant fans.

If all of them check out OK then it's more than likely your Radiator Fan Control Module has fried.

(I hesitate to say this, and note the word, SOMETIMES ((meaning NOT ALWAYS))

Sometimes you can tell by just looking at it.

You can find them on Amazon for around $25.

Now of course it's supposed to do it automatically by itself so something is broken. And Volkswagens are very complex if I were you I was just run a toggle switch from the battery inside the car and then a fuse and a wire that goes to power the fan so you can turn on and off manually anything else is extremely complex to do when your system is obviously broken cuz it's supposed to do that by itself anyway