Ok, you may have to do some wiring, here is what I remember from last time.
For power you will have to connect: Red wire to red wire for power while driving or key in listening to radio, yellow to yellow for constant uninterupted power for memory etc, black to black for ground, Orange, or Orange/white to the same for illumination and I think blue to blue for amp relay kickon.
For speakers, they are colored the same way, just match em up.
If you already have the newer JBL in hand, look at the wires coming out of the back, compare locations in the harness coming out of you dash, you may need to rearrange some, I cant remember.
Now the amp, if you are going to mount it in the wall where the factory does, you will need the full wrapped cable that has the plugs attached already. If you dont you will have to run each and every speaker wire to the location.
If you have to have factory, I would try to find a base model radio with a cassete player, I think they made them, this amp is NOT worth the time, effort and money its going to involve. Right now all your speaker wires are going into the dash, so they will have to be rerouted to the amp, which is a major pain.
It can be done tho, if you are motivated enough. Like I said, I had the amp in mine, but the speakers were toast and I already had a badass cd player, so I stripped my trucks inside bare and put down new sound deadner and new wires, and took the factory amp out and put in a 5 channel with a stealth sub and new 4ways.
Anyways, I would just forget about factory and buy a aftermarket cassete player and buy the adapters at walmart, that way you can just wire the adapters into the new radios wire harness and then just plug it into the trucks dash harness/s and your done. Its up to you tho.
Edit, just thought of something, if you still have the factory speakers in the truck, running this amp will probably blow em up, the base model speakers are not rated for the amps power, that plus age will tear em up quick.