g-gal,
I got the cheapest Stebel horn at autobacs. The $18 one is out of stock. Damn, that's more kool as its red and can be seen thr the vent. Next cheapest is $24, black in colour but sounds slightly deeper.
If you are going to DIY, its cheap loh. The mechanics is probably going charge you more for the installation than the horns itself. Have to remove the front bumper to access to the horn area. For Yaris, the original horn didn't need extra power from the battery. So, I need to install an additional wire with a 20A fuse to the battery. Its all described on the box itself.
For instructions on how to remove the front bumper, search on-line. Should be able to find them quite easily.