Odd FOB and alarm behavior

Hey everyone, I’m hoping someone here has seen this before or can point me in the right direction.


I have a 2018 Tundra Platinum (non-smart key, physical key) and I’ve been dealing with some weird behavior related to the FOB, lock system, and now the alarm going off randomly.


Here’s the history:

• When I first bought the truck, pressing the FOB lock button once would make the truck beep twice, with a short delay between beeps (not typical Toyota behavior).
• Pressing the lock button a second time would not cancel the headlights—they would just time out on their own.
• I assumed this was just “normal for this truck,” even though it was different from other Toyotas I’ve owned.

Later on, I installed a 2021 Tundra OEM head unit using a harness from AutoHarnessHouse. I also installed an aftermarket amp, bypassing the factory JBL amp. After this upgrade:

• The FOB started behaving normally—1 beep on lock, and pressing lock again would turn off the headlights, as I’d expect.
• But then it started to randomly switch back and forth between the old double-beep behavior and normal single-beep behavior.

Now it’s escalated:

• The factory alarm has started going off randomly when the truck is parked and locked.
• A stereo shop confirmed that when I press the FOB lock button once, the truck receives two lock signals—it audibly locks twice.
• No aftermarket alarm system is installed.
• The MID doesn’t show any doors ajar.
• Battery is strong and all grounds seem clean and tight.

It seems like something is duplicating the lock signal or confusing the BCM, but I’m not sure if it’s:

• Something in the aftermarket amp wiring or grounding,
• A miscommunication from the head unit/harness,
• Or a problem with the door lock switch or control module.

Has anyone else experienced this? Any help or ideas would be hugely appreciated

 

Instead of using the fob to lock it. Have you tried just hitting the lock button on the door then closing the door? Or using the key to lock it? I wonder if it will still trigger the alarm randomly.