I'm going to just answer this question easy enough as I can.
Yes, You need a kit for most explorers. Most of them it seems have a low signal from the radio, that goes to a factory amp behind the passenger side in the cargo area. If you're self installing the radio, you don't NEED to buy any special wires or harnesses if you don't care about putting the stock deck back, however there are kits for it.
I just replaced mine, and here is what I did. Note: this may not be the BEST way, and I'm sure I'll hear about it.
Bought Stereo.
Pulled the bezel around the gauges and radio (unplug the 4x4 switch, will probably require the drivers door open to get out, and tilt the wheel down)
Get your dash kit if you want (I've always made my own dash kits using wood or extra plastic, so this is what i did here) The radio will have 3 plugs on the back when you remove it. a Small one, a Big one and of course the antenna.
The one with the small wires runs from the Radio to the "factory amp" in the back cargo area. the bigger wires are the power, ground, power ant (Amp power, ground etc here too)
So basically what I did was 86 the Small wires near the dash, and run new speaker wires from the radio area, under the passenger side carpet, to the back cargo area and spliced the wires into the factory wires already running to the doors. I did it this way for two reasons. 1) Wiring kits can be quite spendy, and there was not many for sale near me. and 2) I did not want to run new wires through the doors. You can fit your new speaker wire (which I got at Wal-Mart for 5 bucks) behind the ashtray, under the carpet in the passenger foot compartment, into the side panel, down to the floor and back under the seats into the cargo area. once all the wires are there it is as simple as just splicing the wires from the amp (the bigger ones) to the wires to your deck.
Doing this this way instead of just splicing the small wires to the big ones will improve signal and will ensure you have good sound quality.
OMG EVER, THATS AWSOME but uhh...how do you get the factory amp plug(s) out?!
GOOD QUESTION. I was DREADING after reading this forum..this part. OMG EVER YOU NEED TO PULL ALL THE PANELS IN THE BACK ZOMG..
Yeah...not so much... Start by (CAREFULLY) pulling out the "cargo net" thingy on the passenger side wall in the cargo area. if you break it, no big deal, you can just add some or your own tricks to secure it back in, but they break REALLY easy. once you have this out, you can look in the hole after removing the trim and the "box" part and you will see...surely and snugly about a foot in is the amp, with a plug on the top and bottom. and probablly some spider webs, dirt, and a dead mouse (if its like my truck
).
With that box out you can see the amp. now, open the cargo gate, unscrew the trim panel holding the carpet down across the back of the truck. lift the carpet unscrew the 2-3 screws under it,
Now: around the panel you will see little circle shaped plugs, pop a few out till the panel comes out (CAREFULLY) once you have room, just unplug the top plug and leave the #$#$# amp in there, I pulled the plug over top of the panel and did my work, than slid it back under once it was all done. radio install with amp kit included took me 30 minutes.
enjoy!