Car alarm/central doorlocking on a Dedra

[Jelle]

Hello,
I just bought a silver '91 2.0 Dedra and I am very happy with it. Beautiful car and drives great.
Only thing that does not work properly is the integrated remotely controlled central doorlocking/car alarm.
When I close all the doors with the normal key the central doorlocking works ok and the alarm is not activated.
When I use the remote only one door closes, and very shortly after that opens again.
The alarm is activated, but the doors are open! When I press the button from the remote again, nothing happens with the central doorlocking, and the alarm goes off.
What could be wrong?
Has anyone a manual or workshop manual which describes fault-finding for the remotely controlled central doorlocking/car alarm?

I think the alarm/central doorlocking remote is built in by factory ....

Regards,
Jelle