I scoured junkyards for a while figuring out how to put one into my 81 242, so here's what I've learned. You're more likely to find one on a 240 GL, maybe some GLT's, and 740 GL, GLT, GLE's and 760's.
There are two different types of trunk release motors, one fits (pre-86?) 240's with the circle type lock. Its a while plastic motor with a long rod connected to the lock. You should be able to ignore this one, it won't fit your lock. The other fits 240's with the larger square chrome trim piece around the lock, and 740's 760's etc, not sure what years. Incidentally this one uses the same quasi-rectangular black motor that is used for the power locks on most 240's. Essentially you mount the motor just to the right of the lock, there's a pretty obvious spot for it, small metal bracket involved, a junkyard car or two should supply you with everything you need plus show you how it works. The motor has a little accordion arm, which extends/retracts as the motor runs, this attaches to and moves the circular white plastic piece on the inside of the lock, which disengages it. Wiring is simple, two wires were all I had to deal with on my 81, one to ground, one to the switch in the glovebox, wire from the other terminal of that switch to the fusebox. I think yours is similar. I tapped the power lead from the trunk light (not going to use them both at the same time anyway) and grounded to the trunk light's ground too if I remember correctly.
Make sure to grab one of the glovebox switches, they do have a cool metal plate behind them that has a little pictogram of the trunk.
Again I looked mostly at 78-87ish 240's, so they may have chaged the glovebox or trunk before 91.
I've used my opener maybe a dozen times since putting it in a couple years ago, but for the time and energy I put in, its worth it.
Good luck.