The Collaborative International Dictionary of English v.0.48:

Wrecker \Wreck"er\, n.
   1. One who causes a wreck, as by false lights, and the like.
      [1913 Webster]

   2. One who searches fro, or works upon, the wrecks of
      vessels, etc. Specifically:
      (a) One who visits a wreck for the purpose of plunder.
      (b) One who is employed in saving property or lives from a
          wrecked vessel, or in saving the vessel; as, the
          wreckers of Key West.
          [1913 Webster]

   3. A vessel employed by wreckers.
      [1913 Webster]

WordNet (r) 3.0 (2006):

wrecker
    n 1: someone who demolishes or dismantles buildings as a job
    2: someone who commits sabotage or deliberately causes wrecks
       [syn: saboteur, wrecker, diversionist]
    3: a truck equipped to hoist and pull wrecked cars (or to remove
       cars from no-parking zones) [syn: tow truck, tow car,
       wrecker]