What are you using for the vehicle integer?

localPlayer.vehicle.id (I check the native stuff on https://gtamods.com/wiki/)
Using the same integer for vehicle engine, which is working.

Also tried the client-side scripting function provided by GTAC:

addCommandHandler("taxilight",function(commandName, args){
if(localPlayer.vehicle.taxiLight)
localPlayer.vehicle.taxiLight = false;
else
localPlayer.vehicle.taxiLight = true;
});

Getting the following:
Error: can't define property "taxiLight": Object is not extensible

The error line is the 5th line of this snippet, btw.

I'll take a look at the native soon.
vehicle.taxiLight is only available on GTA IV right now, hence the "GTA IV Only" at the top of the wiki page for it.

My fault, sry!

The integer for the vehicle arg needs to be a GTA ref ID. The GTAC element.id is not the same.
You can get the ref ID using element.ref