Signal lights do not work

Sounds to me like you have one or more fuses that may need to be replaced.

It might be the switch inside the steering column . It is part of a multi-function assembly that they make for nearly every car now.
It controls the turn signals, either the wipers or lights, the center stop light , and high and low beams. You can fix it yourself, make sure to disconnect the negative ground wire from the battery, you don't want the airbag going off ......

Do it again . you'll find the problem

make sure they installed the ground wire properly

check both fuse boxes,, One is under the dash inside the van, the other is under the hood,,

You only replace the one flasher you need to replace the turn signal one to its also located under the dash if you did that already replace your turn signal switch

The brake lights as well as all the others run threw the turn signal switch . replace this and it will solve your problem.

did you change the flasher relay ? it controls the flashers and the hazard

Have you checked the turn signal fuse? If it is good then i would replace the flasher.

could be a few reasons...first check trailer wireing harrness..if none or ok then replace the curcit board in the tail lights