Nigel070

You probably installed an ELS-Enabled vehicle. You will need this mod to use the lights correctly. Alternatively, look for non-ELS vehicles.

It means you need to direct SparkIV to the correct installation folder where GTA is installed. This is the root GTA folder, so it should be steamsteamappscommongrandtheftautoivGTA IV since you have Steam.

Are you using SparkIV (found here, recommend using latest beta)? You need to use SparkIV to edit vehicles.img and then import the car mod. You'll need to make sure vehicles.img isn't marked as read-only. If it is, disable User Account Control (preferably, since it gets in the way a lot with mods) and mark vehicles.img as NOT read-only. Then try Spark again, making sure to run it as an administrator (right click, run as admin). Either do that every time, or right click spark iv --> properties --> compatibility --> always run as admin.

You can't add car mods using Steam itself (that's what your thread title implies, anyways). You need to use a utility called SparkIV to open up GTA's data files (vehicles.img, specifically) and import car mods.

There are no mods can be installed with Steam. They all have to be done manually, opening up the GTA folder and then importing the data from the mod, based on what the mod readme file tells you to do. Same thing is true if you have the disc version.