Purchase a keyless entry which has a 3rd button, usually used for opening the trunk (~$30) . Obtain a solenoid used for locking/unlocking cars without a door key mechanism. (~$20)

Wire the keyless system normally to operate the vacuum system (your option to ignore if desired)

Wire the trunk open switch to the driver's door solenoid so that pressing that switch opens only the driver's door. A relay (~$8) may be necessary.

Purchase a whistle to be used in case of electronics failure.

__________________
1993 MB190e 104k