The same fuse blows immediately whenever the circuit is switched on.
A short to ground downstream of the fuse draws excessive current.
Suggested test plan
- Never fit a larger fuse.
- Measure fuse-output resistance to ground.
- Disconnect sections to localise the short.
- Repair the harness and re-test with the correct fuse.
Safety: 12 V systems are low voltage but can still cause burns and fire. Disconnect the battery before working on wiring and never short a battery terminal.
Fuse blowing is a symptom, not the fault. Increasing the fuse rating risks a harness fire.