*Wiring temp sender to kick fan in?*

2003 Fiesta 1.25: had problems with the fiesta overheating even though temp gauge says normal temp on dashboard. After box ticking all the usual culprits (relays, fuses, isolating fan to test it works which it does etc) found out it's the ECU ignoring any messages from the temperature sensor. Decided to bypass the ECU by cutting through the top radiator hose and fitting an aftermarket inline sensor adapter and temperature gauge with sensor. Has anybody fitted one of these so that it works the aftermarket gauge as well as kicking the fan in when the engine starts to overheat. Please could you give me some advice as there are no instructions or wiring diagrams and electrics aren't my thing or if anyone knows of any YouTube videos that are actually useful that would also be appreciated. Thanks.

This might sound like a silly question, but what -- specifically -- makes you think it's overheating?

I know this post is old but i've done this quite a few times on cars that have electric fans with no computer. In the mid to late 80'e GM had what looked like a temp gauge sender but was a switch to ground that controlled the cooling fan relay. It had common thread that would fit in the intake manifold or water passages in the block. So i ended up with a temp sender in one water jacket for the gauge and the switch in another water jacket for ground control of the fan relay. Basically copied GM's fan circuit they used in the 80's.