My process for the front speakers would be:
Check the fade is not all the way to the back in the headunit settings - considering that it was resetting itself every time this is unlikely to be a problem.
Cut the speaker wires on the loom immediately after the radio harness connector and temp wire in a speaker to confirm the headunit is working correctly and the speaker actually works.
If it does then there must be a break in the loom somewhere. Might as well run new wires at this stage with upgraded wire.
If they are component speakers then you also need to check the crossover.