You should always check you have a good ground connection firstly.
This can be achieved by locating the nearest ground wire check for corrosion usually a greeny blue colour clean if necessary. Next hook up a multi meter check to confirm the power passes through.
You would be aiming to confirm that power actually passes from the item that's not working.
This is quite a simple process for anyone who tests circuits a lot but can be quite daunting to others who are unsure.
For me I would first check bulbs do they have a complete connection.
If one is out its probably that.
Check the wiring on the clusters check the circuit on the clusters for corrosion.
Check the grounds at or near to the clusters.
After those checks test do they work or not.
Then give the guides a good read.
Proceed to check each connection with a multimeter (if you have one)
That's the method of approach that I would use.
Here I have linked a guide to tail light wiring fix should be very similar on all models.
And also a link to how to check tail lamps. With a multimeter this is a great method to test circuits. As you can confirm the power is passing through or not. Also how much so lets you know if you have good or bad connection.
Tail lamps for cars, trucks or motorcycles can be easily checked for proper operation using a standard multimeter. These tools allow you to diagnose problems with vehicle electrical systems by determining where the problem with your lights may be at. Rather than just checking the terminals...
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