Does anybody know how to install an amp to the stock stereo. Thanks

I think we're gonna need more info than that. What exactly do you want this amp to do? I assume you want to wire up a amp to power a sub off the stock headunit but maybe not. If that's the case you can usually tap into the speaker level outputs from the stereo using either a hi-level to RCA converter or if your amp has hi-level input capability you can go that way.

How do you wire an amp for a sub when you are using the original speakers and factory head unit? A friend of mine wants me to do it for her and I am not too sure. Thanks
Ben

Ok so what you need to do is first determine if your amplifier has speaker level inputs on it. If it does it'll look like the terminals that you hook your speaker cable up to but they'll be marked input. if not you'll need one of these (http://www.crutchfield.com/S-n82DFWj7Sc7/cgi-bin/ProdView.asp?s=0&c=11&g=721&I=101PL2&o=m&a=0&cc=01&avf=N) for this to work. Second you need to run power and ground to your amplifier. Make sure you fuse the power lead off the battery. Now you need to find a ignition hot lead so that you can connect the amplifiers remote turn on to it since our radios don't have power up antennas you can't use that wire.

As far as hooking up to the actual head unit I assume this is going to be installed in the rear of the truck so find the speaker cables going to your rear speakers and splice into them then run your spliced wire to the amp or the line output convertor. This way you should be able to still hear your rear speakers and the sub amp will work too. Now since you will have absolutely no control over your sub at this point besides the volume knob I'd suggest buying an amp with a remote bass control such as a MTX or a pheonix gold amplifier as most of their sub amps and a lot of their 2Ch amps have inputs for the remote bass knob.