I want to construct a motion detector that will fire lights and a horn on a car grill and fender for a halloween effect. I only want it to last 3 -5 seconds then turn back off. Can you help me with a design?

Depending on the motion sensor if you put it on the test setting it should only come on for about 5 sec I would go to home depot and read the directions from a few and see if it tells you how long the test cycle is.

I would use a radar sensor(detector), many have 2 fields, get close and it triggers the warn away, closer and it triggers the alarm output, you will need relays to handle the load of the headlights/lights and the horn.
You could add a wireless remote to trigger remotely. Altronix make delay relays of all sorts.

So does SSAC. You want a non-retrigerable delay on make timer so the timing interval has to elapsed before it is re-triggered.

A motion detector designed for alarm systems and not walk lights MIGHT work better in this instance.