Car just randomly starts blowing alarm ~ happens in the middle of the night while I'm sleeping ~ at church ~ all the places that it is just not very nice to have a car horn blasting!

I am just upset with the fact that I had done a tune up. Replaced rear rotors, ignition wires, spark plugs, changed the oil and the ignition switch. The problem never went away. The truck makes a clicking sound while the THEFT light tries to blink but it's like it cuts off. we took it in for a diagnostics check and they said that there is a misfire on cylinder #6. Anyone got any hints or advice as to where I can start.