Hi Steps.
You have a Mazda truck - take it to a Mazda dealer.
It will cost a bit more for the repair, but you will get warranty on workmanship and spares.
You can pay around 300 baht for a diagnosis, and then decide to let them fix the issue, or go it alone.
As I found out recently - it's a false economy trying to find a decent Somchai garage, and then buy oem parts to save a few baht.
Good luck.