DrGrommel

Did you get engine sounds to work on the vehicles in question? If so: Try out siren mastery by Albo and add lines for the add-on cars. That gave some of my add-on cars sirens (audio) even though they had no proper audionamehash set. Could be a workaround for your issue. 
 
As for lights I gave up on trying to use carcol lines from a custom dlc folder and started editing the global carcols.ymt because I never got the dlc meta to work as well. 

Check out custom visuals in the misc download section. It's a good alternative to radiance and within the download there is a separate taxi sign fix workaround with instructions. 
 
You can use that regardless whether  you wanna use the rest of the mod or not.