the battery is in the trunk. the trunk is closed. the key isn't opening the trunk.

You've got a car like that and you can't afford a locksmith?

Take out the back seat.I have done this on a Mercedes but on most cars you can just pop up the lower seat cushion and remove perhaps 2 bolts on the back and crawl thru to release the trunk. But then if you own a Mercedes pay someone to do it for you.

Hook up the battery pop the trunk and then unhook the battery again

with the key?