This is my first mod for GTA 4, so please don't expect much from me xD
This is purely just a test, I used photoshop to design the new texture and inserted it using SparkIV. I have tested it, and taken some screenshots so you can have a look before/if you download it
I hope to get some more basic textures done soon, and I'm currently attempting to redesign the billboards, too.
Enjoy
Update:
Made a terrible mistake in which I did not realise that both of the taxis in game have adverts... It's just random selection when they spawn.
I thought that the second taxi had the small taxi light on, and the first taxi carried the adverts on top. But I was wrong... So I've uploaded a second archive with the both of the taxi textures in, and an updated readme.
Hope the files appear online soon xD