I have had the text message from Stevie (and don't think I did the mission), yetI can't get any cars from him, nor is he in my phonebook. I need to do his missions because they are the only thing left I have to do for 100%...

Is there anything i need to do before i can do his missions?

no just go to your messages and find the one for stevie. he'll give you an address for where it is then just find it then take it to his garage

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Should the car marked on the map? Because it's not...

no, they are not marked on the map at all. You need to find it based on the description in the text message. He will usually give you locations, street names, towns, or land marks to look for.

nice
ne good cars u have to rob

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yea some are pretty hard to find.
but you could go to this website
GTA 4 Liberty City Map

then just search the name of the car you're trying to find.
it will be really easy to find them if you use this

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Thanks guys =]