I was attempting to replace a dead battery in a 2003 E220 cdi today. I took the existing battery out to take to Halfords with me, to make sure I got a suitable replacement. When I arrived home to fit the new battery, the boot wouldn't open.

I've read a few threads on the topic. Some say that I can use the emergency blade key to open the boot, others say I'll need to hook up the positive terminal in the engine bay to another car using jump leads.

Firstly, I don't believe the boot should have been locked when I disconnected the dead battery. Secondly I suspect I might not be able to open the boot because it's super frosty outside. I can open the doors, and was able to open the bonnet though. Could it just be frozen?

If not, I've tried the "insert the blade key, turn counter-clockwise and lift the handle" using quite some force but it won't budge.

I was going to try the approach of hooking up the car to another using jump leads into the positive terminal in the engine bay / engine block but cannot find the positive terminal. Can someone describe where it's located please?

Any advice from community members would be much appreciated.