Yes wrecks can kill you in UT3 and therefore they can in Ren X as well. Of course like Demigan said, it depends on a few things like how it hits you, how fast it is going, how much health you have, etc..
Just this week I was playing some VCTF and killed a Raptor or Fury (can't remember which) that was directly above me and the wreckage squashed me, giving the guy a kill after I killed him. (And I tell ya, it was really hectic before I killed him because he was trying to squash me anyways so I had to kill the guy, just too bad I couldn't run from the wreck fast enough.)