Polo 2007 Thermostat/Cooling fan.

[Rebelervi]

Hi. Member of family just passed his test and purchased a 2007 1198cc VW which was over heating. He was advised to change the thermostat, which did, but bottom pipe to radiator seems cool, as opposed to top, which seems significantly warmer, after temperature gauge at normal running temp'.
The interior fans are not blowing warm air either.

To test a three piece thermostat, can I use similar method to alternate?
Is it best to remove whole elbow , to change thermostat?

Many thanks.

[Rebelervi]

PS.
Also, on the main earth cable to battery, he noticed a thin black wire , which leads nowhere, what is this for please?

The cooling fan fuse, he was told was located , in fuse panel, side of dash? But manual map doesn't correspond?

Please advise.