No antenna, no radio reception

I would check the connections on the grid, if they are ok you have to live with yagi effect.

for the poor reception have you plugged in the antenna wire into the radio

Check your connections on the antenna again. Am usually gets decent reception. sounds like your antenna connection isnt fully connected either at the stereo connection or at the actual antenna connection.

Heated rear windows have a high voltage load, quickest and easy resolve would be to install an inline suppressor to where the aerial plugs into the back of the stereo, available from all car audio store, it will improve your reception also.

Any car stereo shop should have what you need. Might be a special antenna adapter for certain vehicles.

First start with pulling out the radio and make sure the antenna wire is plugged in. The last owner might have had an aftermarket radio in it and when they put the factory one back, forgot a wire. To remove it remove the trim ring (two screws I think) and then the radio screws.

There is not an easy answer for this one. Some troubleshooting needs to be done. I have seen the Rockford Fosgate system in many of the newer Sentra's. Try hooking a known working speaker to a nonworking position. This will tell if it is a speaker problem. If the problem persists, this could be stereo or amplifier.
Terrible reception troubleshooting, a bad antenna will give poor FM and no AM. If both have bad reception, this is a stereo problem. You can remove the stereo from the dash, place a small pick tool or screwdriver into the antenna jack and test reception. If the reception problem clears, this is a antenna problem. I have seen car detailers pull the connector from the rear window antenna and forget to plug it back in too.

Problem: Radio Reception is bad to non-existant. AM is usually worse than FM.

Reason: Antenna connector has come loose, and needs to be reconnected. This happens when a passenger pushes the carpet toward the firewall and disconnects the plug.

Affected Model Years: 1997, 1998, 1999, 2000, 2001, 2002, 2003

Permanent Fix:

1. First check to make sure the antenna at the back of the radio has not come loose.
2. Remove the plastic cover on the front passenger door threshold held in place by three pressure clips.
3. Peel back the carpet on the front seat passenger floor.
4. Locate the coax antenna cable running along the side.
5. Reconnect the two pieces of antenna that have come loose.
6. Roll the carpet back, and re-install the threshold cover.

Use a dedicated FM T-strip antenna avialable from Radio Shack or even Wal-Mart