2003 Tacoma when I open either door the horn sounds for 45 seconds then dies off. I pulled the horn fuse for now...
Would the door sensor or dome light be tied into the horn circuit?

 

A problem well-stated is a problem almost solved.
We do not know the history of your truck and/or alarm install. At this point, so you're in a better position to troubleshoot it.

 

Basic truck, no alarm system, no upgrades everything is factory 2.4 liter,auto 4wd
no recent work done to it.

 

Still not enough history. DID IT DO IT BEFORE?

 

Maybe the wiring for the horn is somehow loose or out of position whenever the door is opened? Can do a google search on wiring schematics or in a fsm.

Batt not too old? Other issues? Mileage? Accident history?

 

Yes, find schematic. Find where wires for horn come near wire for door-open switch (switches ground) and see if there was an after-market connection (hacked) or a short between the two,

 

My 01 Runner will set off the alarm if I lock it up and one of the doors is not fully closed or not closed correctly. I am the 2nd owner of this runner and it is as stock as the day it left the factory with no aftermarket or accident history.

Even with the horn disconnected, you should see the head lights flashing which makes the truck think it is being stolen. To de-activate the Anti Theft Mode, you need to use the key Fob or lock and un lock the doors with the key. It is a frustrating issue on mine as well.

I would check and see if you doors are getting a tight fit when closed and see if that is part of the issue. I do not have to slam my doors to close them but I do have to make sure I get them firmly closed.

 

Gotta be Gremlins !!!

If your sure there is no alarm maybe this is just your 4Runners way of greeting you .

The fact it shuts off leads me to it being an Alarm or parts of one if it is a short causing this it should blow to the door switch is in the open position

Unless someone tried to install a Alarm made some of the connections found it to much work and gave up.

Then over the years we have seen very strange electrical work !!

 

Been a few days Have you figured anything out??

I am curious !!