Here are the wires to hook to. for the remote you can get a fuse tap and wire it into the fusebox where the radio fuse is and run your remote from that or i know some boxes come with a remote you can hook to. this way it will only send power to the remote wire when your explorer is on. as for the ground just make sure you have a good contact point and you should be ok.
Left Rear (+) Gray/Light Blue
Left Rear (-) Tan/Yellow
Right Rear (+) Orange/Red
Right Rear (-) Brown/Pink
Umm.. I'm not sure which HU your talking about, but for every one I've ever used, the remote wire is for using on the remote terminal on your amp. In your factory audio wiring harness you have a wire for constant 12v+ to keep your memory, and a wire for ignition, which turns your HU on and off when the ignition is in the ON or ACC positions. There is also an illumination wire which dims it when your lights are on.
Again, the remote wire sends 12v power to an amp as a signal to turn it on when the HU is on.
Edit: As per the original question: I've always just put a screw into the body somewhere to use for a ground. Just make sure you scrape away any paint and get a good metal on metal connection, just as rileyrs said. Silicone is also a good idea as he said.
Second edit: I think I misunderstood what mike was saying. Are you saying that you could get a fuse tap to use for the remote wire on your amp, for using a factory head unit? After rereading it, I think that's what your getting at. Only problem, isn't there constant 12v on the fuse? Wouldn't a better idea be to splice into the ignition wire for the HU and use that so that the amp turns on when the vehicle does? I'm still not really sure if that was what you're trying to say, since in the original post jordan said system, I figured he meant he was also replacing the stock HU also.
Third edit: Damn, I really need to get my thoughts organized before posting... I second rileyrs also on getting a wiring harness connector if you are replacing the HU. I've cut the factory harness before a couple of times, and its do'able but it takes alot longer than just plugging in the connector and spicing the wires color to color with the aftermarket HU's harness.