ok you will need a flash lite crawl in there and you will need a pair of good pliers there should be ether a clip or a nut on the back of the key tumbler if it is a clip take the pliers and pull it off it should fall out and you will be able to see the latch now if it is a nut looosen it up till it falls out
Try a lock pick or take off the main part of the lock so that it doesn't need a key like pattern to get in so that you can just use a screwdriver or if you just have a smaller flat head then you can probably just use it in there just as you would use a key.
If you can get into the trunk form the back seat great, if you cannot make to to the aft end by the truck lock then you will have to drill out the current lock and install a new one.
If you can get to the lock from in side the truck then using a stiff skinny flat blade screw driver slide the c-clip off of the back of the key lock cylinder, it will be against the metal of the trunk lid.
You can try to remove the 3 bolt on the plate that the lock grabs but I doubt you can get to them with the lid closed.
You will most likely have to replace the trunk lock cylinder.