Where did you find a 73 SL with the battery in the trunk? All the ones I ever worked on started the rear battery with the onset of catalytic convertors in the engine compartment in 1975.

As to the lock. Those cars have a vacuum locking system and often the vacuum locks work when the key won't. I presume you have lost vacuum. You might be able to go to the source vacuum line under the hood and apply vacuum (suck on it if you have to - you will turn blue), and then work the locks.

In cases where nothing else works we have removed the license plate and with a second car sitting beside, we make a small hole that we use a coat hanger through. The second car is to see where to go inside the hole.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician