The airhorn amp and siren speaker is the hot combo. But, it you want to spend less and still have fun, the way to do it is double your horns. The big caddies and buicks have 4 horns versus the normal 2, a low and high tone.
This his how they get loud and melodious. The short answer on the titan as add two more stock horns, or horns from anywhere, or maybe these:
http://www.griotsgarage.com/catalog....2520&SKU=77573
Put'em in there on the driver side. Probably can use a stock bracket or make one. The real step up in noise though is to direct wire the horn to the battery extra positive terminal with big guage wire and use the stock wiring to trigger a fat 30 amp relay. This provides a lot more current to the horn, and thus, more noise to the citizens of the earth.
This wiring can all be under the hood with no wiring needing to go into the cab. Use a 10 guage wire with a 30 amp fuse holdler built in. run that fro mthe extra postive terminal on the battery to a relay mounted to the fenderwell. Run two 14 guage wires out from the relay, one to each set of horns. Y off at the end to give positive power to each horn. Ground the horns from the other horn terminal. run a thin any-guage wire from the original horn positive hot wire to the relay to be the trigger. Ground the relay with a 14 guage. Pull the ear protectors out of your range bag and call momma out to try the horn for you. It will be damn loud and you will be the monster of the blacktop. Your wife may favor you later in the evening.
juma