I have a 2008 silverado 4x4. I recently started noticed when locking the door with the remote keyless entry, the horn would honk but it was very week sounding. It progressively got worse and now does not honk at all. Now the driver side door will only occasionally lock with the remote or even the electric door lock. Most of the time I have to manually lock the door. The horn does not work at all. It is almost like there is low voltage. I have replaced the battery but it has not made a difference. I have also checked the fuses. What could cause this problem?