Sorry for the perfunctory thread title, wanted to push it past "Dead Battery + Anti-Theft"
I'm the third owner of a..... well-loved.... maintained-ish '06 w/a slushbox.
Been emphatically Googling all day.

Drove it home from work last night. Got up this morning, starter "phwrrd" once, dash lights dimmed, then it was dead.
1. Battery was under warranty, didn't even get it checked, just swapped it.
2. Hooked up new battery. Headlights came on and horn immediately started screaming. Google.
-- Most results suggest an anti-theft system has tripped.
-- Note: This car does not have the security system indicator/reset button to the left of the steering wheel, nor is there a button under the dash.
-- Pulled the horn fuse and started experimenting, tried:
-- Lock & Unlock door, with fob and key and switch (more on that later).
-- Replaced battery on fob.
-- Turned ignition to 'on' five times. Held ignition for 30 seconds.
-- Nothing is on inside. There is no telltales, no lights, power switches aren't doing anything, key-in-ignition indicator isn't buzzing.
-- There's power to the engine bay: Headlights are coming on whenever the battery is connected, and horn has no mouth but would otherwise be screaming, but interior is dark.
3. New battery had a slightly undersized negative post,
-- Note: (it's possible O'Reilly's gave me a different model batt... fits the shelf perfectly, that shouldn't make a difference though should it? As long as the size is right the only thing that could change would just be slightly different cranking amps right?. I'm just spitballing)
-- Replaced both cable ends, sanded all surfaces shiny and smeared dielectric grease on everything. Thought perhaps poor ground was screwing up the interior somehow, nothing changed. (I'm beyond spitballing and into the realm of pure fantasy).

So google is turning up nothing more detailed than "Oh, anti-theft engaged".
Truck is in shop getting front-end work done. Had access to my step-daughter's car for parts today but she will enthusiastically split for the weekend.
Situation turning critical, any advice anyone?

 

Sounds like it was the battery from the first part of your problem. This has happened to me in the past when swapping batteries and I just had to click the unlock button the fob to disable the alarm and everything was happy. Try disconnecting it for a half hour and then reconnecting and see if it's any happier after the ecu resets.

I feel like you're doing way more work than you need to on this and possibly cause more issues in doing so. Sounds like you might have also tripped the interior Room/Cabin fuse, and possibly other fuses by now.

Also make sure the steering wheel isn't locked and you are in park. Just some things to rule out on why it might not be starting.

 

More drama: Just pulled all the small things from the fuse box and found that It's blowing ignition and ETCS fuses... repeatedly. Now I have no more spare fuses.

Which would suggest... alternator problems?

As for other:
1. Car battery has been disconnected for awhile, (didn't want to just leave it sitting there), and reconnected several times while trying to resolve.
2. Fob doing nothing, even with a fresh battery.
3. Wheel is unlocked, car is in park.

Edit: Thanks for the reply, btw.

 

Ooo... I did get the wrong battery. Masculinity reduction. Terminals were on opposite sides.
Got correct battery, replaced all burnt fuses, got interior lights now, still no crank. Will chew on it tomorrow