Flat Batteries On A Mercedes-Benz C-Class
If you have a C43 AMG the battery lives in the boot. The previous generation C-class (1999 vintage) has a boot lock which tends to corrode which renders it unusable. Normally this is not too much of a problem because there’s a button on the key-fob which opens the boot, and there’s also a button on the centre console in the car to release the catch.
I recently found myself in the position where the battery had discharged completey, due to a prolonged lack of driving the car. This means that neither the key-fob nor the internal switch will operate (they are both electrical).
Don’t panic! Pop open the bonnet, and on the passenger side (mine is a right-hand drive model) near the bonnet hinge there is a black plastic cover. Open this and there’s a small rectangular plastic cover under which is a fairly substantial terminal which connects to the battery in the boot. Attach your battery charging equipment here 
Full details are on my personal blog here: The Beast Lives.