I noticed that when pushing the 'LOCK' button on the (wireless) key that all doors, except the trunk door, lock. Manually locking/unlocking works.
I decided to take off the trunk panel and to check the voltage that is applied to the motor that handles the lock - when locking it's -1.7V and when unlocking it's 1.7V (or the other way around, forgot, sorry). I think that is way too low. I checked the motor and it works - I applied 5 volts too it from a USB power supply, but I think it's a small 12V motor as it sounded a bit "powerless" at 5V (or the USB power supply couldn't provide enough current, but the supply is rated for 2Amps). Thinking 1.7V would be enough I didn't want to destroy the DC motor so I applied 5V, nothing more, it still works, I put it back in, got back to the car, put the panels back on, and I'm back where I started.
I tried looking where to find the fuse box and wiring diagram for the SEAT Cordoba 1.6 version, so I could check which fuse is responsible for the trunk lock, because I think the issue might reside there, but I can't find it anywhere.
How would I go about fixing this issue so the central lock will work on the trunk door too? Any tips? Resources to find wiring/fuse diagrams?