I am still convinced the factory amp in your truck is in the passenger side kick panel, if there is an amp under the pass seat I would assume it is aftermarket. you say the plug that has the speaker wires in it plugs into it but it is unplugged, how sure are you that these are the speaker wires? do they go to the speaker, the deck, or both? have you tested these wires to make sure they actually go where you believe they go or are you just matching colors up? if you have a camera some pics may allow us to help you better.

 

whoever did the radio ran 4 sets of wires from the head unit to under the seat to an aftermarket plug kit to the factory plug, which is the plug that hooks to this amp under the seat. the amp is factory it says land rover on it. i could use this setup but i dont have constant power anywhere, that i can find, so when i turn the truck off the unit loses all the presets. the factory plug has more than just the speaker wires in it. my thing is if they used a kit to connect the speaker wires why not for the head unit?

 

well if they ran the speaker wire then they just bypassed the amp (this is what I would prefer to do anyways) now for your constant power if you can't find constant power behind the radio (and you have not only tested all the wires back there but also checked all the fuses) then I would simply run a new constant wire (with a fuse inline) from the ignition switch and be done with it. If you suspect any of the speaker wires behind the deck maybe wired backwards (if the audio sounds at all strange) then you can use a AA battery and "pop" the speakers, positive to positive and negative to negative and the cone will move out towards you.