Righto, first things first, you need the xvehicleenv128.tga reflection map that comes with SA. It's in the vehicles.txd, or if you're spasticated, you can find it here.
It looks like this:
Please don't attempt to texture this to your car, as it is in gif format. As long as you have THIS file loaded into your z3d file, you won't have to add it to the car's txd. San Andreas will simply read it straight out of the vehicles.txd, which is handy.
Now we move onto the second stage. I am going to give this little box thing an SA-style reflection map:
It's not a car, but your car will work the same way. First thing you have to do is right-click the box's object on the right, and click 'Properties':
A dialogue box will pop up. You want Mesh > Vertices > Format > UV Channels, which you set to 2:
Now click OK. Your box/car/whatever can now cater for two UV channels, the first of which will be whatever you have mapped to it, and the second of which will be the SA reflection map.
Next we need to set up the material in the Materials Editor. Press E. Now find the texture that is going to have this second refmap (probably your main body texture) in the drop-down list. Scroll to the very bottom of all the options. You're going to enter a new texture layer, so make sure a row has it's box ticked on the left, and click the 3-dotted button. Click Add at the bottom of the Textures Browser, browse to your copy of the xvehicleenv128.tga, click Open, then click OK.
You should have a texture layer like I have above, except the material will probably be something other than Default Material, and you'll have other texture layers (like one for your body map and UV Channel #1). Now for your texture layer, where it says Apply as:, click Edit...
As shown above, change the Apply texture as: option to Low-level environment map, and the Use UV mapping: option to Manual UV #2, then click OK. Click OK again to exit the Materials Editor.
You're almost finished now. Select all your objects in object mode (Select > All), and go Surface > Mapping > Edit UV. Tap the space bar to turn selected mode on (should be red at the bottom of the screen), and click in either the left of right viewpoints (depending on which you have enabled - make sure it's one of the side views though). A dialogue box like this should pop up:
Choose the Remove mapping option, and make sure only Channel #2 is ticked. Then press OK.
Now again click on the side view with the Edit UV tool, and this time leave it on Create/Edit UV. Set it to Edit UV Channel map #2 and Already mapped vertices behaviour to Generate New:
Your model should appear in the UV Mapper window like so, after you click the Materials button at the bottom of the UV Mapper window and click your material (you can also click Objects here and select which object you want to edit):