A few years ago I installed a fuse holder inline with the constant +12v to the horn relay next to the drivers side headlight to power halo's in the headlight. Of course I tested for power with a test light at the horn relay power wire and it simply lit up. I'd pull the halo fuse to prevent draw when the car wasn't in use. No issues for years. Fast forward to last weekend.
I blew the tail light fuse, pulled all fuses one by one in the drivers footwell box to find the culprit then realized it was in the IPDM. Replaced the blown tail light fuse. Then, upon installing the aforementioned halo fuse in the holder the center (bumper) horn (not wheel well unit) began sounding!
Now when I test the power side of the halo fuse holder with the test light the center horn sounds. I tried multiple grounds points. If I put the halo fuse in the holder the same horn sounds and the power led for the halo module lights.
Both horns sound when activating at the steering wheel but only the center horn sounds when locking with the key fob.
