Bumper Crop - Georgena S. Sil

Drivers of four different vehicles lined up behind one another at a red light, completely unaware of the amusement they created for pedestrians passing by. Each vehicle sported a different sticker on its rear bumper, and each driver could read only the message on the vehicle directly in front. Can you deduce each vehicle's place in line, model type, and humorous bumper message?

1. A large truck (which wasn't first in line) spewed black exhaust somewhere in front of the vehicle whose bumper read MURPHY WAS AN OPTIMIST.
2. A sports car with blaring rock music was somewhere in front of the vehicle whose bumper warned RADIO ACTIVE.
3. None of the drivers could read the parcel-laden vehicle's sticker which chortled I GOT MALLED!
4. A van was ahead of the station wagon but behind the vehicle whose bumper proclaimed TEAR ALONG DOTTED LINE!