Can you get into the boot of a CLK320 via the back seats

Nope. But fitting a tow bar shouldn't stop you opening the boot... Does it unlock with the key? Don't forget to try both ways because as I remember one way is a kinda dead lock so it doesn't unlock on the fob.

one could take the seat out there should be 2 two little tabs that you should be able push in on the seat bottom and lift up on the lower seat cushion it should come up Fairly easy with that out you should see two bolts at the bottom of the seat back between a 10 and 13 mm bolt undo the bolts and lift straight up don't pull out words push up from the bottom the seat back should push straight up the back of the car. then remove the sound deadened panel if there is one. It should be wide open . you could try a double male end wire that comes with the jump box plug one end into the box the other end into your cigar or accessory socket turn the key to accessory to where the radio will play then push the trunk release button. I Hope that this was helpful.