Originally Posted by wstar

Very strange. I wouldn't even look at the ROM, I'd bet on a wiring mistake during the install of the G3's and oil cooler. There's wiring for the horn up there, and depending on how you do the install(s), you may have to disconnect it to move it out of the way. Other things up there include wiring for stuff like ambient air temp sensors, washer fluid bottle, radiator fans, crash sensors, hood latch sensors. They're all unique connectors that can't be plugged into each other I'm pretty sure, but something went crazy up there, and the horn + wiring for it *is* in that area. Maybe all it takes is one broken ground wire somewhere to cause an odd fault state that produces your behavior, who knows.

You might see if there's any correlation with any of those systems: do they all work (rad fans run when car hot, ambient air temp on gauge roughly correct, washer fluid sprays, hood latch warning if you roll off with it open, etc)? Do any of them cause funny horn sounds?