The guy left the after market Alpine speakers in it, and they definitely all worked when I test drove the truck. But I do have a couple of other after market speakers from my old truck I can try.
Searching on TRS, people were saying that there is a so called factory amp right in front of the right rear speaker. I found what I assumed was the amp in that spot until you said something about there being no external amp.
I thought that when a speaker blows, it just gets all static. Mine are not working whatsoever.
EDIT: Found a wiring diagram. http://www.therangerstation.com/tech...dio_Wiring.pdf
It shows that there is external amp. Guess tomorrow I will be getting the meter out and try to find out what is wrong.