Tap into the speaker wires and feed the amp inputs from there. There are devices called Line Level Converters (LLC) that will take speaker levels and convert them to RCA signals. Some amps have that feature built in.
Yeah, the LLC's will work, but the generally sound pretty crappy(as compared to aftermarket HU). And you have to be careful of where you mount them, because they are usually very susceptible to EMF, and will create nasty engine noise.
Also, does the radio have RDS, and is that why your staying away from an aftermarket unit?
I recently bought the truck, and have an amp, and sub from my old car.... I like the stock look, i have a cd and tape .. i had no plans to upgrade anything stereo wise, but i like to mess around. how bad are talkin that llc's sound? what is EMF and RDS.