I had the same problem with an '83 SD that I recently purchased. I tried all the keys which came with the car, but none of them would turn the lock. I looked for a cable release, but the car doesn't have one. I tried sucking and blowing into the yellow line under the back seat, without success. I tried hitting the lock with a hammer and a block of wood. I ended up drilling the lock out, and then finding that the lock was seized. If I had lubricated the lock before trying the keys, one of them might have worked.

From my search of this problem, I learned that some cars alarm systems can prevent the trunk from opening. I looked under the front passenger side carpet to see if the alarm was unplugged, and found that my car does not have an alarm.

I didn't find any bodies, but I did find a good spare tire, and a new set of jumper cables.

Since you don't have the key, have you contacted a dealership to obtain a key, based on the VIN of your car?

If you don't have the key, and your car doesn't have a cable release under the back seat, you might have a problem. If the trunk lock is in the locked position, you'll have to break something to get it open.