Here's a couple of threads that will help...

http://www.toyotanation.com/forum/t67044.html

http://www.toyotanation.com/forum/t67877.html

http://www.toyotanation.com/forum/t67970.html (that one will probably help the most)

 

There are adapters on the market that allow speaker level (amplified) audio signal to be reduced to line level audio signal so it can be connected to an external amp via the RCA jacks. This is usually the most effective way to do it, if the radio doesn't have RCA output jacks on it. Can be done - but it adds some unwanted "noise" to the system. You'll probably never even notice it, though. A good head unit with jacks already there is always the best alternative. Good luck!

 

sorry to say, the only other way to get more power into your speakers is with another amp. infact, if you keep the whole system like the speakers, headunit etc. You're truck is already equipped with a factory amp located behind the rear seats in the doublecab model, and would have to tapp into that in order to add another amp to your truck, but there's a catch. A slight misinstall will make that stock amp fry, so if you want more sound, and youre not willing to go all out, then leave it. Its not worth frying your stock amp. unless someone is willing to take you step by step. I called toyota and they told me the stock amp is in the hundreds to replace.