I have a 1998 CLK320 originally brought to help out a friend. I have been away with work and come back to find the battery flat, I can get into the cabin using the manual key however but it won't open the boot to allow me to get at the battery. I have tried putting power back into the car using a lead into the ligher socket but have had no joy. Can anyone help me out?

Chalkster

 

the manual key should open the trunk to access the battery. turn the key counter clockwise and push the button with the key still turned left in.
that should work unless the trunk latch/tumbler was replaced with one from a different vehicle.