Coolant leaks

Hi

I replaced the radiator of my 1991 Jetta (70 hp engine) three weeks ago because the old one had a crack and was leaky.

The problem with the new radiator is, the system is still leaky! The problem is, where the thermo switch is screwed in coolant leaks out - not much but I'd like that to be proof again, else I could have left the old radiator in the car as well ...

Now i do not know what was the mistake

either I didn't screw it in tight enough - I could have done half a revolution more but then the electric wire would be contorted

the other alternative is that the old metal sealing doesn't do it's job anylonger. I know such stuff normally needs to be replaced every time when opened but since a friend of mine didn't even change the metall sealing of his oil drain plug and the car is still oil proof I though I could leave the sealing on as well.

What would you recommend to do?

Greetings

Thomas from Austria