There are 2 types of signals: Line level, and Speaker level. Line level is a pre-amp signal (1-8v olts) that must be amplified. Speaker level is the output from the amplifier that goes right into the speakers.
The factory head unit sends a speaker level signal to the speakers obviously, and does not provide any line level output. Even if you have a factory amp, the head unit sends a speaker signal. The newer Kenwood tibs may be different.
You can tap into the front or rear stereo speaker level signal, and connect it to a line level converter. The line level converter takes the signal, impedence matches it to show the same load to the speaker, but comes out with a line-level outputs also, and has RCA connectors. You then connect the RCAs into your new amp.
If your amp accepts a high-level, you can just directly use the speaker level signal right into your amp.
However your best option is to buy an aftermatket head unit, which will come with line level outputs. You can geta descent Kenwood deck for around 100 bucks. You will notice a huge increase in quality over the stock deck.