I'm in the process of installing an alarm and I was able to find most things out myself by doing the 12v point tester thing, but I got stumped on the dome light and power locks.

Dome light: I pulled the dimmer switch, thinking that I could just splice into one of those wires to turn the dome light on/off. I found the hot wire and then turned the dimmer up all the way to turn on the dome light. The dome came on like it should but the hot wire went away (all four became cold). Confused, I tried another ground only to find that the same thing happened. Opening a door also made my hot wire go away.

So... How can I turn the dome light on/off by adding one 12v wire without ripping out the headliner and running another power lead to the light?

Power Locks: What's the easiest way to get the power locks (factory) working remotely? The alarm came with two wires that switch polarity to activate the locks. There are three wiring setups in the manual: positive and negative (which just flip current back and fourth to trip the relays as if you were hitting the power lock buttons) and reverse polarity (involves two relays outside of the factory setup). I'm guessing that we use either positive or negative, but I need to know which one. I would also like some advice as to how to do that splice/install.

 

The dome light operates by 'grounding out', not by 'switching on' +12V power. The orange wire is 12V constant, and the white one grounds out, either through the headlight switch or one of the door pins switches.
Not 100% positive on the locks, but I believe they just need reverse polarity to be thrown in the opposite direction.

 

That explains why everything went cold. I don't understand how a system like that would work though. Wouldn't something somewhere have to be supplying the dome 12v so that the bulb can come on? If so, where's that wire? Is there a ground out sensor that gives it 12v that I can tap into?

 

Yeah, the orange wire is always at 12V.

 

So if I feed the orange wire 12v, the dome will come on?

 

No.
The orange wire always has 12V in it.
When the white wire grounds out, either though the door pin switches, or headlight switch, it completes the circuit and the light lights up.