2003 Ford Expedition, Factory Sub to Aftermarket Radio

i have a 2003 ford expedition with factory sub i want to hook up to my jensen vm 9512 head unit. any help would be great

Metra 70-5521 harness will get the sub workin

you need to find the input wires to the factory amp that drives the factory sub. it's probably next to the factory sub itself. once you find the amp i think there will only be 4 or 5 wires going into it. 2 wires will be 12v+ and ground. a third might be switched 12v to turn the amp on. the 2 that are left should be the signal wires. (test each one of these wires with a digital voltage meter, if you are not sure what a wire does leave it alone) if you find the signal wires and are sure of what they are, the next step is to run a single RCA from the blue sub output from the jensen to the factory amps' signal wires that you found. cut the end off of the RCA to expose the 2 wires inside and hook that to your signal wires that you found. solder and tape if possible. this should work but there is a good chance your sub will sound weaker because the sub output from the jensen TVs are notoriously weak.

Or you could just use the correct harness.  The one mentioned above is designed to retain the factory sub.