Been there.
Done that...
On my CL500 ( C215, M/Y 2000 ) there was no Batt+ connection in the engine compartment.
And of course - the rear boot lid - did not open since the mechanical lock (Huf) was totally corroded and stuck. Common problem since no-one ever use that lock.
Instead, Open the driver door using the mechanical key, then open the passenger door. Under the glove compartment, remove the mat and the aluminium plate protection plate. There you will find a Bat+ connection.
Once a charger has been connected and the voltage is on the way up, the remote key or the switch on the driver side door can be used to gain access to the boot.
The Huf boot lid lock gets corroded since there is often a water leak from the 3rd break light.
One way to check for this is if the rear plate lights are black/brown from galvanic corrosion caused by water ingress.
Fixing this requires it to be taken out, sprayed with conformal coating in order to protect the LED's from galvanic corrosion. Then, the gasket may need replacement, OR the the gap may be filled using floating silicone. I hesitated to do this, since silicone is aggresseive against the lacquer, but there was no reaction, and the leak was fixed.
The Huf lock mechanism can sometimes be repaired. This requires disassembling it completely (while memorizing how the lock elements are configured).