12 vlt acc source under hood of 2004 Expedition


Senior Member
Actually, I would say generally that "no one" uses a schematic in the real world of stereo install (just ask Ski). They use a multimeter and basic understanding of vehicle electronics. Getting a schematic takes way more time than just finding a 12 volt wire with a quick underhood inspect.

Nowadays with all the high power amplifiers, they run a cable straight to the battery + post and put a high amp fuse right there, and then run this 4 awg or so wire through a hole drilled in the firewall through a rubber grommet to where the amplifiers are. Then, the rest of the audio system comes off of that point for all rest of 12 volt sources. The grounds are also brought together to one point to eliminiate ground loops as well.

The "fusebox" under the hood is usually a "Fuseable Link" box. It is usually higher amp circuits that then power up the "fuse box" under the dashboard.

You could always just take a wire, put a large end connector on it and secure it to the battery + terminal. But, put a fuse right there next to the battery. Any distance between the fuse and the battery is unprotected and subject to causing a fire if abraded and grounded.

you can also go to the alternator output terminal if that is more convenient.


guys, guys, guys,


I need both a constant 12 volt and an ACCESSORY SOURCE that supplies voltage when the car is running. I need this for the logic incorporated in my car monitor (second monitoring method) described HERE!

I can figure out where to get the constant 12 volt under the hood (there is this large, black, rectangular box with two very large terminals on top that gives me a constant 12 volts whenever I place a meter on it independent of the vehilce running) ;)

Man, this is turning out to be like the "Quest for the Holy Grail"!

"I seek the GRAIL!"

Also, perhaps I should switch to decaff?


Senior Member
100% correct

BSR stay out of the ignition harness!
In the time that this thread took, you could have easily run a new wire from a Fusetap at the fuse panel to your device under the hood, either by drilling a new hole with a grommet, or using an existing route.

Suggestion 2:

BSR if this is on the truck that is in your picture, simply remove the factory radio and tap into the accessory feed. I think it is Yellow and Black 16 or 18 guage.


Skibum said:
If you have to go to the fuse panel use a FuseTap. Never force anything into the socket.

Just an update, I found a fuse tap that Skibum suggested right in my local Pep Boys Auto store! Used that and problem solved (just had to "modify" the outer case to extend the wire outside the fuse box). I now have a working car monitor in my wife's Expedition! :lol:


Active Member
BSR, I do mostly Euro types, but a source you might look for is at the windshield wiper motor. There should be a wire there that stays on in the acc/ignition positions. I would use an appropriate sized in line fuse for the wire that you are using.