Because they faded out I would verify the ground, where the horn is bolted to the body. If that has become a high resistance point then it would act to drop the voltage across the horns. Horns are voltage devices. The higher the voltage the farther the voice coil will move and the louder they will play. They are not intended to be operated continuously so if you laid on them and held it you could have over heated the voice coils and fried them.
If only wives and MIL’s worked the same way…
Thank you
Did you only jumper the positive side from your battery, or did you wire both sides? That would rule out a bad ground. I’d do that before I spent money.