My 560SL has the battery in the trunk, and I store it for several months each winter. The first year I simply opened the battery cover and the lead clips from the charger went to the post terminals. Since then, I wired up a pigtail from the battery that ends in a two-conductor plug; it stays in place full-time and I simply plug in the charger when storing the car. The wires from the charger terminate in the other half of the plug connector. The wire comes out at the edge of the closed trunk, and there's been no damage to the seal and no leakage.
Safety points: Make sure that the charger side of the lead is long enough that the charger is outside the trunk (no desire for 110v wires or active electronics in a sealed space). Wire in a fuse holder on the car side and give it a fuse no higher than the max rating of the charger--3A is plenty. I used the flat rubber type connector usually used for trailers. With that plug, the pigtail has an exposed conductor end when not in use; make sure it's the ground side--no exposed hot leads. And if you contemplate using the charger for anything else, simply save the alligator clips and wire them to a second plug to mate with the one on the charger.

It sounds like a lot of work but is about a half-hour project, with no chance that alligator clips will come off of anything--especially useful since I store at the cottage and the car's unobserved for a few weeks at a time.

__________________
Craig Bethune

'97 SL500, 40th anniversary edition

'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)


SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)