Interesting Electrical Issue

Tractionless

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.

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Toys Not Tots

Tractionless

Fun update

I put a voltage meter on the power side of the fuse holder and get battery voltage. When inserting the fuse now the center horn barely sounds (instead of full on blast like above) and the power side of the fuse holder drops to 7.3v.

With or without the fuse only the center horn sounds when beeping the horn at the steering wheel. In the OP it worked normally. What change did I make? The car sat 5 days untouched, so nothing. Think a ground issue may be the culprit.

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Toys Not Tots

Tractionless

Fixed it after ruling out component and wiring issues while picking through the FSM.

Apparently while "borrowing" a fuse from a good circuit (horn) to fix the aforementioned blown tail light fuse; I didn't replace it within the fuse holder in front of the battery. Being I found the horn circuit is powered from two sources, I only had half the design voltage. This explains losing half of the battery voltage over the halo controller fuse once it began drawing power.

Glad it was a simple solution requiring little elbow grease but nonetheless time consuming due to neglecting the detail of returning a fuse to the borrowed position.

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Toys Not Tots