Quote:
Originally Posted by brucepick
I considered finding the 12V pin on the OBDII connector and supplying a good solid 12V to it directly.
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The horn causing strange behavior isn't all that surprising since it's a pretty high powered electromechanical device (it spits out lots of electrical noise, not just audible). The headlights on the other hand are a pretty stable... They pull a peak of current when you first switch them on, but are a constant load after that.
I would take a good look at your battery and engine ground cables actually. The only power supply wiring shared by the lighting and engine control systems runs from the positive battery terminal to the under-hood fuse box, and from the negative terminal to the body ground point. If there's a bottle-neck that's causing power supply problems, it would most likely be in one of those connections.
Old batteries can sometimes leak acid, which can be picked up via capillary action by the battery terminals and passed on into the battery cables themselves. Once there, it's shielded from the elements and has plenty of time to corrode and dissolve the copper wire. When the cable gets down to only a few strands left to carry power, all kinds of weird gremlins start popping up. The easiest way to detect (and neutralize) an acid leak is to fill a spray bottle with a solution of baking soda and water, then spray it around the battery and cables (so long as the battery's vent covers are in place, the solution shouldn't hurt it). Just like baking soda and vinegar, the solution will fizz up if you hit acid.