Installed a horn, car won't start
 

I just installed a wolo air horn as a replacement so I can actually be heard. The horns work great, but now the car won't run. Most of the time it just cranks, and since it does I'm pretty sure the battery's fine. Sometimes it will start, run for a second, and then sputter and die. The only other change is that I replaced the 10 amp fuse for the horn with a 15 amp. Nothing else was touched. Any ideas?

Disconnect the wiring for the new horn and try starting again.

-then-

Disconnect the car battery for 10 minutes (clear temp memory from the ECU) and try starting again (you'll need to enter your radio code so have it handy).

-then-

Throw yourself at the mercy of the Honda svc dept (you may have fried your ECU).

I don't think substituting a larger fuse in an existing circuit is a good idea. They are sized to protect the wiring and other components. I would have run a new wire from the battery (appropriately fused and correct gauge wire) to a relay powered by the existing horn circuit to run the new horn.

Hope you did no permanent damage... :(

I really hope nothing got fried. I mean, if the ECU was gone it wouldn't even start at all, right? And on occasion it does start and run for a few seconds.
I also noticed that the dash doesn't light up at all now. I can't figure out what fuse might have been blown, and I don't want to pull every single one out to find out if one blew. Also, when I removed the fuse labeled backup (number 1) the radio reset. It's a 10 amp fuse, so I can't imagine the whole car lost power. Is it mislabeled? Can my car have some strange wiring in it?

If I just disconnect the battery is that going to erase the keyless entry/ main alarm codes? The keyless entry still works, and I want to keep it that way.

Throwing a 15 amp fuse where a 10 amp is supposed to be could be your problem too! Once you did that the malfunction light -the one looks like the outline of a engine- should have immediatly indicated on your dash...:D

Right now nothing is lighting on my dash. If I turn on the headlights the light for that illuminates, but thats about the only one. The windsheild wipers work, but on the intermittent and off positions they just freeze, and don't return to the "home" position. The rear wiper also doesn't work, and the pumps for the washer fluid don't work. The turn signals also don't work. But on the bright side, the radio probably would still work if I put the code in.

How badly did I ruin my car?

Unfortunately, you are going to find out, one way or another.

I think you have a good starting point for discovering some truths. I'd try Steve244's suggestions. Disconnect the wiring to your new horn and reset, and see what happens. If everything magically starts working except for the disconnected horn? Then you can consider yourself lucky and move on...

If not? You probably have a bigger problem and will venemously hate the day you decided to upgrade your horn. But at least at that point you probably would know to seek professional help.

Well, I unplugged the horn, and then the battery. While it was unplugged I removed and checked every single fuse. None were blown. When I plugged everything back in it all worked. No idea what happened, but it seems to drive fine now. Thanks.

Wow...that is strange..

What? That's implausible.

Before doing your victory dance, you may want to put the 10A fuse back before something else is blown. There is a reason why Honda engineer put in a 10A fuse instead of 15A.