Hola,
You're looking for an intermediate connection, not the Relay. The cable from the fuel pump should connect to a three plug cable, kind of on the right side of the engine bay near the firewall, that's where the connection to the fuel level sender is. The fuel level cable is the pink one, which, I think is the middle connector. You can test your gauge by putting various resistors between the connector for the pink and the one for the black (ground) 90ohm resistance is full, 0 ohms is empty.
This three plug cable is what you disconnected the cable to the fuel tank from when you removed the fuel tank.
I found this video to be really useful when I fixed my fuel pump and sender:
https://www.youtube.com/watch?v=R4Br1Jq9jEEThis is the link that contains my repair:
https://www.fiero.nl/forum/Forum2/HTML/145778.htmlOne key thing is that I really wish I had replaced the cable instead of cleaning the corrosion. Everything now works, but replacing the cable with the Racetronix Universal cable would have been a much better repair. Not sure if they ship to Spain or not.
[This message has been edited by gjgpff (edited 02-04-2022).]