I have an 85 500sel myself and i don't recall any type of pull chord under the back seat. ( I have been under there before to fix the seat ) all of the locks on my car are vacuume powered and yours should be no exception. If the trunk doesn't unlock when the key is turned and doesn't unlock when the rest of the car does, then what i would look at is a small metal linkage inside the lock system in the trunk itself but seeing as you can't get in there now that would be a bit of a problem. if your sure the key sounds like it unlocks and it feels like it unlocks then a locksmith will not help it's more likely that the small metal lever system is loose or out of alignment. As to opening it... i am still thinking. it may be as hard as cutting open the metal behind the license plate to gain access to the problem, but on the other hand, i could be wrong and there would a cable you can pull under the seat. (REALLY doubt it) my seat is apart right now i'll look for you tonight when i get home. BTW in order to take off the seat, there two 13mm bolts one on either side of the bottom of the seat if you feel adventurous.

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iThink Different
Building an all new electrical system in my
1985 500SEL