You probably have a vacuum leak in the line to the driver door or the driver door has a leaking solenoid. The power lock system is partial vacuum controlled and partial electrical. The solenoids of the two front doors and the trunk have an additional electrical switch to control the vacuum pump so that all other doors get (un)locked.
The line or the solenoid to/in the driver door is probably leaking.
Do you have to (un)lock the trunk manual all the time or is there any chance that you locked it once and therefore have this (un)locking "problem"? The trunklock can be locked manual after which it does not unlock with the power lock system...

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)