My SL600 did that a few times I would have to pull over get out and shut the trunk lid again. Do NOT slam it with a lot of force!! This will almost surely crack your third brake light, which will cost about $150 for a new one and is not a fun job to replace. But you can try closing it with a little weight, that helped mine to realize that it was closed. Once I replaced my third brake light which was slightly cracked when i got the car, I decided not to haul cargo in the trunk and didn't mind since that wasn't what I got the car for anyway. I have other cars for hauling things. But, you can try closing the trunk manually using your weight a bit. just don't slam it.... good luck. If that doesn't work, then maybe replace that switch that tells the car the trunk is shut, don't know the part number or cost...