One thing that is nice about using an alpha channel is that the doorlines can be made non-reflective. Its a subtle detail, but it really looks nice.
Create a new layer on top of the previous one. Now hide the layer we were just playing with, and make sure the new layer is selected. Select the Line tool and set its width to 1.5 pixels (for most of the time, adjust if needed). Now pick 0/0/0 as your color, and trace over the doorlines on the texture. Also trace those black strips that run along the roof, as those are non-reflective in real life. Here's what I get after doing that:
To make it look nicer, create a new layer below the 2 we've been painting on, and make it solid black.
Now merge all 3 of these layers we created, but do not merge with the base texture.
Now we can put this in the alpha channel.
Photoshop users
Select the layer that we made, and hit Ctrl+C to copy it. Now go to the Channels tab and select Alpha1 and hit Ctrl+V to paste into that channel. Now click on RGB, go back to the Layers tab, and hide the layer we made. Go to File-->Save As... and save this as a 32-bit TGA (I like to use compression), over-writing the original texture.
Paintshop Pro users
With the layer we created visible, go to Masks-->New-->From Image.
In the box that pops up, choose "Source luminance" and make sure that "invert mask data" is unchecked.
Now go to Masks-->Save to Alpha Channel, and in the box that pops up, click OK if there is no existing alpha channel. If there is an existing alpha, the overwrite it. Now go to Masks-->Delete, and click NO when it asks if you want to merge the mask with the current image.
Save this as a TGA, overwriting the existing file.
NOW ONTO THE WHEELS