This is a simple script i wrote because i need some of this functionality for my other still unreleased mod "crime does pay" (sorry will take some days still - had only recently time to work on it).
To install, just copy to your GTA4/Scripts directory. Obviously you need the .net Scripthook (you know where to get that by now i hope
To start the mod, you have to hit shift + N. This will load the saved cars, spawn them, mark them as "needed for mission" and attach a blip to them.
To save a car, just hit "Q" while inside a car. This will prevent it from getting deleted (i hope), attach a blip to it and write it to a file on save. If you already saved this car, only its data (colour, health, position) will get upgraded.
To save all cars to file, hit shift + Q. This will write all data into a file inside your gta4-install-folder called fcars.ini
On top of this file (generated on first save) you find also a value called "MAXNUMBEROFCARS". You can control the number of safeable cars by playing increasing/decreasing this value.
Sorry at the moment no customizable keys - i wanted to release this quick because some of you might also like the funcionality in a stripped form. If you really cant stand the setup - just change it yourself.
As always for my work, not only a compiled script but also the sourcecode (.vb) is included! only use one of both files (.dll / .vb) at a time!
Feel free to reuse parts of my code in your own work or reupload my mod, but ALWAYS remember to credit me and include this readme!
Edit: For temporary download-link go to linked gta-forums thread!