Ok I've successfully got the Tahoe working in the policeold1 slot with no issues. No special troubleshooting needed. Clearly there is something wrong on your end which is preventing it from working.
I'll walk you through installing this tahoe step by step to make sure you do everything right:
Firstly, DELETE YOUR MOD FOLDER.
There is no way around this. I have no idea what you've done to cause this issue but without physically being at your PC there is no way for me to find out, so the only way to be sure the problem is gone is to delete your mod folder. This obviously means you will need to re-install any other mods you have installed, but if you don't there is high chance you will run into more issues in the future.
Now that your copy of GTAV is clean, we can install the model.
Rename the model's files as per usual to
policeold1.yft
policeold1.ytd
policeold1_hi.yft
Then download the carvariations.meta and vehicles.meta at the bottom of the post.
Place all the files somewhere where you can easily find them. A good place would be a new folder on your desktop.
Open up OPENIV, and at the top of the window click on the button labeled 'Edit mode'.
Go to update\x64\dlcpacks\patchday12ng\dlc.rpf\x64\levels\gta5\vehicles.rpf, Then click on the red button that says 'Copy to "mods" folder'.
Once it's finished copying the file, click on the little green cross on the top bar. In the window that pops up, navigate to the location where you have placed your new files (which will be the folder on your desktop if you followed my suggestion), select all three of the vehicle model files (policeold1.yft, policeold1.ytd and policeold1_hi.yft) and click 'Open'. The files should now be visible in the window, probably at the bottom of the list.
Now go to update\x64\dlcpacks\mpluxe2\dlc.rpf\common\data. Once again click click on the red button that says 'Copy to "mods" folder'.
Once it's finished copying the file, right click on 'carvariations.meta' and click 'Replace' on the pop up menu. In the window that appears navigate once again to the location of your new files, select the 'carvariations.meta' file and then click 'Open'. The original carvariations file should now be overridden by mine. If you double click on the 'carvariations.meta' in the main window it will show the contents of the file in a preview window. The first entry should be 'policeold1'. If it is, then you are good to go.
OPTIONAL STEPS: The vehicle will now appear ingame and have functioning lights, but if you try to drive it at speed it will flip constantly when you try to corner. To fix this you will need to give it a better handling line by editing the vehicles.meta. I've already done this for you.
In OPENIV, go to update\update.rpf\common\data\levels\gta5. Once again click click on the red button that says 'Copy to "mods" folder'.
Once it's finished copying the file, right click on 'vehicles.meta' and click 'Replace' on the pop up menu. In the window that appears navigate once again to the location of your new files, select the 'vehicles.meta' file and then click 'Open'. The original vehicles.meta file should now be overridden by mine.
Now open GTAV and test it out. The Tahoe lights should be flashing, but they will be dull because you won't have any lighting mods installed. If you're having trouble seeing the lights, drive to a dark area like a tunnel or change the time to midnight, the lights should be visible.
carvariations.meta
vehicles.meta