Have an 06 f150 amp behind back seat right now running small wire from amp up to factory harness about 75-100 watts. Don't think I want to run new wire thru door molex plug. Would it help at all to run larger wire from amp to wire harness or will it not matter?

 

Under sizing wire when using high amounts of power and long runs can worst case scenario cause the wire to burn out, short, and potentially start a fire due to the wires resistance. But that would require a very high amount of power or an extremely long run which you don't have.

Depending on the actual rms power you're feeding to the speakers as well as the ohm load, you'd most likely worst case see less power at the speakers than what the amp is putting out due to wire resistance increasing the impedance somewhat.

So can you do it? Most likely, yes. Should you do it the right way and run 12-16 gauge speaker wire all the way from the amp to the speaker? Again, most likely yes.

 

I guess my question is should I leave it as is or run larger wire as far as I can before going down to factory wire

 

In short, it's all or nothing for the wiring.

Your circuit is only going to carry as much current (amps) as the smallest gauge wire in the circuit can carry (the chain is only as strong as its weakest link). If you are planning on using smaller gauge wire than what is already installed, that's fine, just don't go do a bunch of extra work routing wire unless you make it so that ALL the wire is the same gauge. If you re route a bunch of thicker wire and then leave a 6" run of smaller wire, the max load that circuit will be able to carry will be limited to whatever that smaller wire is rated for. You'd be wasting your time.

Again, as mentioned above, you'll most likely be fine, but it entirely depends on the max current you'll be pulling.

An easy way to find out would be to rig a multimeter (set to amps) so that one lead goes to the black wire (coming from your amplifier) that would plug into a speaker (i.e. sub woofer) and the other lead goes to the actual terminal on the speaker. Power up your stereo, crank the volume, and see what the amperage is. The component most at risk in this test, is the wire itself.

 

The factory wiring in my 13 screw is 18 ga and I am pushing 100 w rms at 4 ohms thru it to my front door speakers and its just fine, has been for a year and a half.

 

Thanks that is what I was wondering

 

Not exactly. Assuming your amp puts out a full 100 watts rms per channel at 14.4v where it's rated at you then have to take into account with other accessories in the truck on, your amp is no longer receiving the 14.4v it needs to run at full power. In addition, you'll rarely ever have it at listening volume where you're running the amp at full power (assuming you don't have your gains jacked up and are clipping all day long). So while your amp can put out 100rms per channel, your probably sending closer to half that to each speaker.