DrGrommel

Well one solution would be to rename the files of one of the vehicles to another stock vehicle's name that you don't mind replacing and just add those renamed files to the vehicle.rpf you are using. Keep in mind though that handling, LODs and other stuff might be very different between cars and that the vehicle will most likely not fully work as intended if you don't use the slot the vehicles's creator edited it for (if you don't drive the vehicle yourself it might not matter much and still work ok enough though). If you wanna do it properly you would have to edit the global vehicle.meta-file so that the lines for the slot you are using are matching the vehicle's intended slot. Or at the very least change the handling-id so it fits the vehicle you are installing.... this is what I meant with tedious :P

Yea those packs tend to go overboard and add way too many vehicles with way too many polygons. Plus with the latest update many cars from older packs have issues with their lights now. I'd love to have something like this that works, but for now I just started replacing assorted in-game vehicles with models that work and are not too taxing on performance. It's a little tedious but it also avoids other issues that add-on cars can cause.

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.