hello to all, i am dumb founded about a long red cable that looks like it starts at the battery and runs along under the carpet all along the body heading toward the rear of truck.I traced to about half way toward the rear of the truck until i got tired.My truck is a 91F150 lariat 5.8.If you can tell me what this wire is for i would thank you so much.

 

Depending on how the truck was spec'd ,,,
Could be a cable to supply power to something at the rear of the truck/
Does it have power and about what thickness would you say it was ?
The diameter /gauge wire could help tell what it was for. Maybe some one added it .

 

You already traced it half way and you didn't see any inline fuse or switch, so the other end of it needs constant power, whether or not it connects to something or is terminated at the connector.
1) Let's assume that device is not driven by a relay and rule out that wire is going to terminal 30 of a relay. With your key out of ignition switch (because it doesn't need switched power), check all connectors/device/anderson connector/plugs and observe your findings.
2) Get a voltmeter and measure their voltages and observe their respective readings
3) Disconnect that red cable from your battery terminal post
4) Use your voltmeter and measure their voltages again

Anything has voltage on step 2 but not on step 4, then you have found it.
And apparently it's not a factory wire.

 

thanks,i do believe it is factory,it is run along with all the factory wires,funny thing is when i bought the truck ....used...this cable was not connected to anything but it is coated with a rubber black coating about 2 ft. down the cable to signify it is a negative cable and it goes into the truck body very factory like and no more black coating and all you see is 4 gauge red battery cable going to the rear of truck inside cab.

 

Single battery above, dual battery below.

 

There are a few items that seem to cause some confusion.
First ,,,IT LOOKS LIKE, have to be sure !!
Second -4 GAUGE, pretty hefty stuff
So,,,,,,,,,,,Does it have 12 volts at the back end Yes/ NO ?
If it is a supplemental ground , OK.
If it has power (12 volts) and not protected from contacting a ground anywhere keep a fire extinguisher handy

 

I guess I will continue to trace this cable and find out what it is connected to.Weathrr is bad right now but rest assured I will let you know what I find.Thanks to every One

 

Found IT......the wire was the hot side of a past stereo system, and the reason everything looked factory was because the wiring was run by a professional.Now the job of cutting this junk out of my truck.Thanks to all and have a great day.

 

Thank You for the update.

Something you might consider,,,,,,,,,,,,,,,,,,,,,,,

With the price of copper these days and a professional job , you might want to reconsider what you call junk, lol.
If you wanted to do anything off the back of your truck that required power, you have it available for free. Just consider disconnecti9ng that cable from any power connection and look at it as ,"Money in the bank". It doesn't eat and you never know when you might need a convenience back there..