Where is the taillight relay located?

Sounds like you are on the right path. The relay for this is in the driver side junction block. They also run off the tail fuse also so this should be checked as well.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

I am attaching the wiring diagram below for this. I would suggest starting with the license plate bulbs as this is a simple circuit and would be the best to start with because if you figure this one out then the others will most likely be fixed as well.

Please run through this info and let me know what questions you have on this one.

Thanks

Thanks very much Kenney, that really helps a lot! In the end it was a fuse problem. The fuse schematics in the owner's manual didn't match the box. I reworked many combinations methodically and got lucky. Thanks so much. You're wiring diagram really helped my thinking!!