Roller blind doesn't work every time on W140 600SEL
 

W140 - 600SEL 1992
The car has the original electric roller blind for rear window. This blind will not work every time I need it, but time to time it will work perfectly. I wounder why.
Could it be a bad relay? If so, where is that relay located? Thank you :)

I searched the forum but no results for my problem.

check the fuse box located on the rear right compartment and clean them with a contact cleaner and pencil eraser or replace fuse, also check front fuse box located on the left side engine compartment near the left firewall where the brake systems are located, sometimes it may be the switches too, but they can be repaired.

Thank you cool. I'll do a check and refer back.