Could someone please tell me how the current flows through the engine bay?

Does the current go first through the relay and then branches off to the fuses, or is the current split to the fuses without having to go through the relays?

I have a spooky problem with my car, the only power getting to the car is: horn works, passenger side power seat works, security system lights work. Otherwise nothing else is getting power. I bought a relay tester and that tells me all of the relays are working good. Looked at all the fuses and they show they are good. Before I pull each fuse to test it am I going in the right direction? Is there a major link that could be causing my problem?

Also what does a diode do and how do you test them?

I know I have a bad wiring battery cable (according to the dealer) and have been waiting over two months for a new one to come in. The car worked fine before, just a small drain on the battery was the original reason for getting a new one. I thought I could drive using the old one, but then this problem came up.
Thanks

 

Alternator (back connector under the rubber plug) cable goes to the starter solenoid terminal, far end of battery positive goes to the same post on the starter solenoid, and then you have the cable off the battery positive post that goes to the engine fuse block. Engine bay items for the most part, come off the engine fuse block.

Inside the car, engine fuse block to BCM (where the fuses are inside the car as well), then distributed from there out the BCM fuse block to the internal modules.

As for ground, battery to both the chassis and engine block, with most module grounded back to the frame.

So here we have the terminal for the alternator to battery cracked off the soleniod to start with, and the ground bolt to the engine block for the battery end of cable right about the starter.


Upper to lower engine fuse blocks,


BCM and note that there are fuses/relays in the covered boxes below the BCM and to the right of the BCM as well.

 

Just to add some helpful info to Dano's post with regard to the picture of the BCM: The blue connector on the left is C1. The blue connector on the right is C2. I used a magic marker to identify them on my car so I don't have to look it up.

 

Middle red connector with 10mm bolt in the center of it on the BCM, that is the power connector wiring in from the engine fuse box, to the BCM to start with.

So if you check with Tech II and BCM is showing low voltage, that is the one you need to deal with cleaning and greasing, as well as the BCM ground points as well.

If you have low voltage down line of the BCM on these modules, then into C1 and C2, as well as the splice joints and ground points instead.



Last one, and gauges for voltage (dash and DIC), have nothing to do with the gauge cluster voltage, and is telling you the voltage of the ECM at end of line instead. Hence gauge is digitial, and is getting it voltage signal digitialy from the ECM, to display its voltage.

And to add, to really check the system, Tech II is needed, since it will tell you each module voltage reading in the first place.
As for under the hood in regards just to the charging system, lights/Hvac/radio/radiator fan off and engine idling, should have 14.7V at the alternator (back terminal under the rubber boot) and it body used as the ground), should have 14.5V at the battery post terminals, 14.5V at the engine fuse box terminal and alternator body used as ground. At the DIC voltage, should be at 14.1~14.3V.

To add, if you have a relay, and fuse on the same line, the Fuse will be down line of the relay, to protect the relay and circuit from direct ground shorts. The diodes are to allow current to flow only one way through them, and even basic multi meter will have diode setting on it, to check direction of flow through the diode.
Hence current through a diode is only in the direction of the arrow,

 

Can someone post relevant wiring diagrams for him please