The 555 works extremely well - I had it in a '83 T-bird years ago. Also, instead of abusing your sound system, get some super horn tweeters (Radio Shack) and drive them w/ a pair of MOSFET's. Put one side of the tweeters to power and the other side straight to the MOSFET's. You can use a pot to tune it for the most irritating sound possible.

The only problem for me was my alarm crapped out and the siren went off while I was driving - I got about 1000' before I had to pull over and get out!

You could also do what a friend of mine did, glue razor blades to the back of the cage. He had his share of problems, though. Came out to his car one morning and the interior was covered with blood! That evening the stitched up would-be thief came back and set his car on fire.
[[[Lesson to be learned - do not seriously injure the would-be thief!]]]

Phil
Blue #010101261