How about a fake radio show where whoever calls in first gets to send flowers/gift to their significant other. Then have your wife be the 'winner' and when they ask who the gift should be written out to have her give the name of some other guy like, "My lover Bruce Bowen." or something of that nature where she thinks your wife is a cheater.
I vote for some kind of voice that seems to be coming from the rear of the car. Set her stereo so it works on the back speakers only, and make the recording so that the voice sometimes comes from the left speaker only and sometimes the right speaker only. You could also record your own voice and pitch shift it so that it sounds really high (like a child's). Then make the "child" calmly and quietly say really disturbing things over and over (no screaming - it will sound fake and still might make your mother-in-law drive off the road). Another idea - make the recording sound like her stereo is somehow picking up a cell phone conversation where two people are discussing something really awful (murder) or really kinky - maybe even both.
A fake recording of a phone conversation where her daughter is supposedly talking to a friend about having someone elses baby.
How about a thump-thump noise followed by a dog yelp? She'll probably get out to stop and see what she hit, see nothing, then get back in the car and drive off - until she hears it again two minutes later -------- Or, some police sirens and then a helicopter hovering overhead, blaring out, "You in the Toyota Avalon, pull over." "Turn off the car, roll down your window and throw out your weapons." "Put your hands out the window where we can see them." "Proceed to the nearest McDonalds where the manager will search you." -------- A child's voice saying from the back of the car, "Wait! You're not my mommy! 911! 911!"
If you do the trunk one, make sure you turn the "fade" function all the way back to the rear speakers.
