I have a 12v negative ground, but what I did was ebay some cheap pizo buzzers that were two wire. I put two in, one for left and one for right. Tied the black grounds together and attached them to chassis ground. Attached the reds to the turn signal wires, which were not wired correctly based on the original diagram, but found them with trial and error. The pizos are very loud, so even with my bad hearing, I can't miss them. I tied them to the steering column with tiny black tie wraps, which is where I spliced them in to the turn signal wiring. I do like the audible signal now and because they're a bit loud, I turn them off as soon as I can.
I did a search and here is the url. Mine looks like the first one in the list, but mine was from China and shipping was free from what I remeber.
http://www.ebay.com/sch/sis.html?_nk...72.m2749.l2658
If you buy two you can reverse the wires for positive ground and they support a range of voltages. They were only a couple of bucks each at most, so maybe worth just doing that.