A solenoid is a sort of tube with an electrically activated magnet on the inside (if Im not mistaken). It will have a long cable protruding from one end which is tugged when the solenoid is activated, thus opening the lock which you connect to the end of the cable.
I fitted one myself when I had my tailgate smoothed, basically I used an existing hole on the inside of the tailgate and drilled another to keep it horizontal then attatched the cable to the locking mechanism. It came with a button which I fitted to the dash. Fitting is pretty simple, routing the cable to the dash not so, so take your time. If I remember rightly I paid roughly €54 for the lot.
Ned