I just finished adding an aftermarket solenoid to my '99 Camry for use with my alarm.
I have not seen a real nice breakdown of the job w/ pics.
1. Get out your panel popper and pull down the trunk lid liner. Self explanatory... no pic needed.
2. Grab the bracket for the solenoid, measure the distance for the mounting screws, and drill the 2 holes behind the passenger side tail light (this is pretty much the only place that the solenoid will fit inside the trunk skin.
3. Rout the cable from the solenoid thru the trunk lid over to the latch mechanism. Remove the 2 bolts holding the latch to the trunk and let it hang down slightly. Loop the cable around the arm of the latch that the bar from the key lock mounts to. Mark the length of the cable necessary to reach (and loop around this arm) and then loosen the retention screws on the soleniod. It is much easier to install the crimp bolt on the cable with it hanging out of the car. Then remount the solenoid in its bracket, and unclip the green clip that locks the bar onto the latch, and loop the freshly crimped loop around the arm before reattaching the lock rod. Make sure that you do not leave a bunch of slack in the wire, it only has 1/2"-3/4" of travel. Heres a pic of the latch when you are done:
4. Scrape the paint out from under one of your bracket mounting holes and attach your ground strap there. Rout your + wire down the arm of the trunk and into the car. Make sure that you use a relay to power the solenoid (they draw around 10 amps and will burn up the circuit board of your alarm unless it is intenally relayed). Heres the completed pic:
