I've done it different ways at different times.
-For non-post-apoc I'll just brush with matte varnish (avoid the windows unless you want them cloudy) and I might drybrush a bit of tan along the bottome edge.
-For weathering as-is, Usually I'll brush on matte varnish to kill the shine. Sometimes I'll give them a light once over with an abbrasive of some kind (a soft foam sanding pad is one option) to kill the shine, but you have to make sure you don't go through the paint.
-A bunch of dirty washes is a classic way to weather and it works well.
-One neat way to really weather it heavy is to brush-dip the body (after disassembly) with minwax Antique Walnut or Army Painter Strong Tone. In the places where it pools too much drybrush a bit of burnt orange and it looks like dirty rust. Then matte varnish the whole body. Then reassemble and drybrush tan all over. You can see the results of this method at the bottom of this post.
http://chicagoskirmishwargames.com/blog/2012/03/16/28mm-on-cheap-sci-fi-bikers-and-pos/A bunch of other car examples in the post.
I've had a different experience than others regarding stripping. I've not found alot of wire marks under the paint. In fact, many cars (especially Hot wheels) seem to have paint so thick that when you strip back the paint you end up with alot better detail.
They mostly lean towards post-aopc but here's a few other posts of cars that I've repainted in various ways.
V8 Interceptor.
http://chicagoskirmishwargames.com/blog/2012/04/22/last-v8-interceptor/http://chicagoskirmishwargames.com/blog/2011/12/21/28mm-on-cheap-toy-cars-as-wargames/http://chicagoskirmishwargames.com/blog/2012/01/08/28mm-on-cheap-post-apocalyptic-vehicles/