This is a known issue that can affect the Liftgate Module (LGM) on some vehicles. I take it your 2004 is out of warranty? There was at least one service bulletin on this that I found #03-08-52-002
1) Check to see if all of the parts of the LGM are dead. a) The rear wiper, b) keyless entry (you said it was) and c) lack of auto-locking when closed. You might also notice problems with your radio shutting off and a couple of other things.
2) You could check your fuses, but if all of the LGM components are not working then a dead LGM is more likely. There are at least two fuses, #3 & #6 which are located under the rear driver's side seat fuse box. Also check the ground from the module plug back to the body, especially if you still have rear wiper control.
3) Unfortunately, this module (if it's bad) needs to be flashed/programmed by a dealer from what I understand. It will cost about $350 +/- for the total bill.
4) Another problem comes when the wires in the boot for your liftgate get cut or split somehow. That's something that can be fixed yourself, if you have the ability. Will have to troubleshoot that though.