Haha, this took a while to type as I had to keep rereading to make sure I was even remotely making sense.
I REALLLY REALLLY appreciate your help. Especially on what to look for, I was getting so confused looking at datasheets and not being sure what was important and what wasn't. Also, I am not LIMITED to mosfets, if there is something you think would be more ideal please mention it. They were just the first thing that really jumped out at me as maybe being able to do what I want. Also, if it helps anything, it wouldn't be too hard to put a LDO 12V Vreg (ie. the sharp PQ12RD2) on the signal line.
First, bountyhunter, this is going to power a ballast (when I said HID I meant the ballast which powers the bulb).
Awesome...thanks so much for the information from ALL of you. Yeah I can ground switch these or power switch these no problem. Is one technologically easier to do than the other?
As for the load profile, it will be automotive voltage for both the voltage source and the signal. The amperage varies some: the start-up amperage can get really high. Case in point: A person on the HIDplanet forums used his labs oscilliscope a while back to test out ballasts and their startup current profiles. He tested a Hella Gen III ballast plus a few others IIRC and measured 60-100A for roughly 200µS at startup. After which, it drops to around 10-20A or so and decreases gradually from there for the next 30-40 seconds, where it reaches its steady-state amperage of roughly 3-4A. Supposedly, newer ballasts limit this initial startup amperage (at the expense of a bit longer warmup time), but I want to design for the worst case, not the best. I don't know if that super high start up amperage is something to worry about or not, I do know that a 30A mechanical relay fused at 30A handles two ballasts starting up just fine.
Does the steady-state current of a combined 4A change the Ron recommendation (though I will definately keep it in mind as I may design part of the circuit to run halogen bulbs or bixenon solenoids).
As for the wattage, did you just use the current (30A) and Ron to calculate that?
Lastly, good idea on searching for one that could be mounted.. It's very common to mount electromechanical relays to the cars body, which should serve as a pretty good heatsink. Though, not needing one of course is more ideal, but that's definately lower down on the priority list. I guess the only problem with mounting it to the cars body would be waterproofing. Though that's something I am sure I could come up with if I had to.