I recently bought a 20VT plus from 1999. The central locking via keys works. It also includes the option for remote (un)locking of the doors, which has not been used in a long time probably.

I changed the batteries of the remote controls. Though still none of the two work. I hear nothing in the car when pressing the button on the remote control. I disconnected the wires on the yellow box, refitted and retried => still nothing.
There is 12V power going to the yellow box.

Any tips for trouble shooting?

In the UK we never received any factory fitted alarms like your M20 yellow box, perhaps because they didn't meet (or were never tested against) the requirements of the Thatcham testing category 1 for alarms that the UK insurance industry expect. On the UK we got either the Scorpion 918U (upgrade, from the Thatcham category 2 standard Fiat CODE immobiliser to a category 1 alarm) or Meta M99 (or M99/T2) which were fitted AFAIK at point of import to the UK. For us, a "yellow box" means the CODE immobiliser box...

Do either of the remotes have an LED to indicate anything is happening when you press the button? Is there a switch or anything on the remotes? If your yellow box is supposed to connect to the central locking can you trace the wiring back to the control box (under the driver's seat?) and see if there's a break in the wiring?

No led on the remotes. There is a push button and i pushed it directly without the casing. It’s not an alarm, just a remote control. No visible brake in the wiring on first sight but i will have a closer look and do some measuring of the part i can disconnect.

Ah, so you just have that M20 remote locking box, also under the seat, and no alarm?

Maybe the guys at Abacus Alarms could help you?

https://www.abacuscaralarms.co.uk/accessories-spares-2/central-locking/meta-plug-play.html

Also, since it is a factory fitted unit, there may be something in the handbook or workshop manual?

I found nothing in the manual which helps so far. Next step can be to trace all the pins with the wiring diagram. Though the real life situation seems to contract the wiring diagram at first glance.
I opened the yellow unit, visually it all seems fine. Unfortunately no relais or led which indicates if it receives information.

The yellow unit is a meta s4416 ls/a

Is that a CR2032 or similar battery inside the yellow box? Have you changed that? (Although I'm not sure why it would need one)

Do you know for sure if the keys and yellow box were ever working together in the past?

Have you checked that the yellow box is getting an earth connection too?

Sorry for causing confusement. The small part with the batteries is the inside of the fob, and these are new. I checked the voltage to be sure.

The yellow box has continuous 12v from the coupe battery. I don't see any batteries inside the yellow box.

I gave up for know, i can do without the remote locking