AND I don't know if any of this is pertinent.

Two possibilities:

1.Do you have a Factory Alarm?
(Red Dot on the Master Key)
and if you have a Factory Alarm AND no key with a red dot...
You MAY not have the Master Key to that Chassis.

With the Master Key in the Trunk Lock,from vertical key position:
One click Counterclockwise is UnLock.
One click Clockwise is Lock
Two clicks Clockwise is "Lock Out" the Valet Key (AND maybe the "Headless Key")

2.My one thought is that you don't have a "Bastardized" system...
BUT, that the Trunk Lock (MIGHT? We hope) should have an Electrical
Switch that OPENS the little Electric Motor that's driving you crazy,
when you operate the key in the Trunk Lock.
ONLY IT'S BAD AND NOT WORKING???

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'84 300SD sold
124.128