I'd like to upgrade my 2024's single horn to two Hellas. I've found the harness to adapt it, but I can't seem to find the specs for the 90105-A0343 bolt. Any good sources for one that's corrosion resistant? I've seen it at Toyota dealers but the shipping is almost 10x the bolt, and my local dealers only have contact (not purchase) forms for parts, though in a pinch I'll have to do that.
Can't you just order it online, or at the parts counter, for pickup at your local dealer, to avoid shipping costs?
Most all hardware stores sell D.I.N. European specification bolts. Toyota uses J.I.S. specification bolts. They have hex flanged heads. The M6 European flanged bolt matches t h e D.I.N. Japanese bolt. In the larger size M8 and above, for example, the head is 12mm hex instead of 13mm in the D.I.N. version.
Maybe you aren't interested because you've already got some of the parts or you really like the Hellas, but I found the 08522-B1010 kit that Toyota makes to be super easy. Contains two horns(1 low, 1 high), the harness, and a bunch of OEM hardware(I only needed the bolt). I was able to do the job in maybe 10 minutes. It was around $75 on ebay when I bought one a couple years ago, but might be closer to $100 today. Just search for "08522-B1010" on the different sites. It's technically a kit for the Japan market, so you need to use a third party like ebay or Amazon to find someone that's imported them. It's about $30-$35 from places like Impex Japan if you want to do the importing yourself. Total cost would be something like $50-$60 including shipping and taxes/tariffs. So it would be cheaper, but would take something like a month or two to get to you.
You could bring an M6x1.0 and an M8x1.25 home from the hardware store, and toss whichever you don't use into the junk drawer for later.
Another option would be to temporarily remove your existing horn(you were going to do that anyway) and take that bolt to the hardware store to find a match. Every store I've been at that sells fasteners will have a panel somewhere that has all the different bolts and nuts glued to it so you can test yours to find out what it is. In your case, you'd go to the metric nut section and try to thread your bolt into the different nuts until you find a thread match. Then use that info to find a matching bolt the same length.
His local Toyota dealership does not sell parts online. The two closest dealerships to me are the same way. You need to call to get a price for any part. I called my local dealer about vacuum hoses for my Gen 3 Prius. They found the parts and gave me prices, but they would not tell me the part numbers. I ended up buying the parts online from a dealership that was 350 miles away, with free shipping, and it was cheaper than my local dealership quoted me.
My final suggestion, if you really want an OEM Toyota bolt but don't want to buy the Toyota horn kit, would be to include the bolt as part of a larger online order from a Toyota dealership. The Memorial Day sale should be coming up in just a week or three and would be a great time to stock up on maintenance items. Oil, filters, wiper inserts, etc. If you get the order up to $75, you get free shipping on the entire order. You could include the bolt and get it for a little under a buck. I've switched over to Valvoline Restore & Protect now that they make it in 0W-16, but you could still probably get to $75 by ordering a few oil filters and drain plug gaskets, a set of air filters(engine & cabin), and a set or two of wiper inserts. Even if you don't use the air filters right away, the engine and standard cabin filters don't go bad or anything if you leave them on the shelf until you need them.