There are two different ways to achieve placing a layer like a car number over a headlight or taillight in Paint Builder.
Method 1: Display Car Parts below layers
One way to achieve placing car numbers or other layers on top of headlights (or other car parts) is to toggle the “Display above layers” option for Car Parts.
With all layers deselected, scroll to the Car Parts section of the Properties panel.
Uncheck Display above layers. This will treat the ordering of all non-mandatory Car Parts layers as below layers of your project.
Method 2: Make a copy of a Car Parts layer
If you don’t want to move all Car Parts layers behind your custom layers, you can make a copy of an individual car part and reposition your desired layer on top of the copy.
To do this, choose Clone as new layers from the Car Parts block in the Properties panel.
Choose the part for which you’d like to place a layer above — like headlights, taillights, or another car part, and clone it.
This will make a copy of that car part into your Layers list. You can reposition that copy anywhere you’d like among your project’s layers, including moving your car number layer above it.
Hide the original instance of the Car Parts layer.
Note that in some instances, car parts like headlights and taillights may be mandatory and force-stamped by iRacing. A general rule for determining which car parts can and can’t be edited is if the part appears in the Car Parts layers list (the list in the top-left of the Paint Builder interface, underneath the vehicle name) — typically, if the layer exists in this area, it can be painted over; otherwise, iRacing likely restricts painting over that area.