The reason for the contracts being written that way doesn't have to do with used car lots using them.
For an outfit as big as Chrysler, you're talking about hundreds of thousands of signs, which will require hundreds of thousands of man hours to remove, replace, and dispose of. So Chrysler contracts with a company to remove the old signs, hang up the new signs, and properly dispose of the old ones, and the contractor has to be able to show they satisfactorily removed every old sign, replaced it, and properly disposed of it before they'll get paid by Chrysler. I doubt Chrysler cares anymore about how their old signage is used than the Datsun and Nissan guys you mentioned. What concerned them was hundreds of old signs laying around busted up, dirty, or in other disrepair at some contractor's property because they didn't want to pay whatever the disposal costs were, which might tarnish the image Chrysler wanted to portray. The purpose of spending millions on a resigning is to erase some message the old signs sold, and you can't do that if there are dozens of old signs lying around dealerships because no one took the time to get rid of them or some contractor decided to pocket the money he was paid to take them to the dump and just leave the signs piled up on his property.
All that said, since there are hundreds and hundreds of old Plymouth signs out there, its obvious there's been ways to get around contractual requirements... and that way is most likely cash in hand.
