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Discussion Starter #1 (Edited)
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Hello guys, did a quick search and found an old topic but the fix is related to an alarm system.
I Don't have an alarm and my 03 Taco is stock with no extra lights or anything, so far all the fuses are Ok, day or night the lights stay on, with headlights on/off, light switch on/off makes no difference.
Only way to turn them off is by removing the fuse for the tail lights or by disconnecting the battery.
Any ideas where do I need to start looking?

edit: runs very rough, almost to the point of shutting off, runs normal on the road
 

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Mambeau / Admin
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The dash lights and corner marker lights (front parking lights) are on the taillight circuit. You've got a short or some other fault that's causing the circuit to be hot regardless of the light switch position.

A bad light control switch would explain it. A persistent current from the Body ECU to the Tail Relay or a bad Tail Relay might also explain it.

Here (attached) is the 2003 Taillight / Illumination wiring diagram.
 

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Discussion Starter #3
The dash lights and corner marker lights (front parking lights) are on the taillight circuit. You've got a short or some other fault that's causing the circuit to be hot regardless of the light switch position.

A bad light control switch would explain it. A persistent current from the Body ECU to the Tail Relay or a bad Tail Relay might also explain it.

Here (attached) is the 2003 Taillight / Illumination wiring diagram.
Thanks for the reply, I'm going to take a look at the diagram, see if I can figure out the problem
 

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Mambeau / Admin
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My #1 suspect is the Tail Relay, but it could be either the relay itself or the light control switch.

If the relay is passing current even though it's not activated (from the light control switch) that would explain everything.

Similarly, if the light control switch is shorting out (whatever ... ) so that there's current continuously flowing through the relay's activation coil that would explain everything.

Also, here's a tip that might save some time in zeroing in on the fault.

The circuit connects to the taillights and license plate light via the BN7 connector (which is underneath the driver's seat).

Locate the BN7 connector and disconnect the line leading back to the rear of the truck.

If this makes lights-on problem stop, the problem lies somewhere to the rear of the BN7 connector.
 

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Discussion Starter #5 (Edited)
My #1 suspect is the Tail Relay, but it could be either the relay itself or the light control switch.

If the relay is passing current even though it's not activated (from the light control switch) that would explain everything.

Similarly, if the light control switch is shorting out (whatever ... ) so that there's current continuously flowing through the relay's activation coil that would explain everything.

Also, here's a tip that might save some time in zeroing in on the fault.

The circuit connects to the taillights and license plate light via the BN7 connector (which is underneath the driver's seat).

Locate the BN7 connector and disconnect the line leading back to the rear of the truck.

If this makes lights-on problem stop, the problem lies somewhere to the rear of the BN7 connector.
Thanks for all the help, following your advice, I was able to pinpoint the problem to the Tail light Relay just as you suspected.
In case somebody runs into the same issue, the fastest way to troubleshoot the Tail light Relay is to swap the EFI Relay (see picture) since the Relay's are the same, my Taco was also running rough so I ended replacing the Tail Light Relay and the EFI Relay, after the replacement the Taco is running normal once more.
The cause of the problem was a cable on the rear tail light coming into contact with the body.


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Mambeau / Admin
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Glad you found the problem ...
 
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