If you fire full-auto at a vehicle with something like a SMG, what happens to the passengers? Do they have to resist the 20P that the vehicle does with their own Body + Armor, do they get the car's armor added on, do they resist the damage that the vehicle took, or is it handled in an entirely different way?

A situation came up in my game a couple of sessions ago where we were hiding in a stationary vehicle. Our enemies saw us as a threat, and fired full-auto at our car. I don't remember how much damage the car took, but it was somewhere around 14. The way my GM ruled it was that we resist the 14P on our own after the car took its damage, but I'd like some clarification on how exactly this works.

Well the way I work it in those situations is the characters resist the same damage as the car but they get their armor plus the car armor plus their body and they get a dodge roll first (at a -2 penalty). Auto fire etc affects both car and passenger so they both have to resist the damage but in other cases (eg. single shot) you need to pick what you hurt, either the car or the passenger.
But what kind of car has so little armor that an SMG is a threat? So long as base damage (5P in this case) plus net hits of the firing character doesn't exceed vehicle armor value, the bullets do nothing, since it is hardened armor. A 14P after full auto sounds like it was just base damage with no extra hits. Since you can't remember exactly, I'd guess the damage was a little higher but still not enough to get past the car armor. Unless of course your car only had 4 points of armor which makes it either a Jackrabbit or Thundercloud ATV (ah that's what type of car fears an SMG). All this assumes no specialized bullets on the part of the baddies.
If the baddies were targeting your characters specifically then they suffer a blind fire penalty of -6 and you still get to dodge (at -2) and get all the car and personal armor. Even still, if the modified damage before autofire doesn't exceed the car armor it doesn't punch through. Also, cars in Shadowrun (and most other things with glass) use polarized glass to prevent magical targeting which also prevent visual targeting. Or they did in previous versions anyway. So shooting at passengers is always blind fire unless they hang their heads out of the car or something. My 2 cents.

Hm...

Okay, I see what we did really wrong now. We were using the rules for shooting at barriers instead of the ones for damaging vehicles.

But, the baddies were loaded with APDS rounds (I think) and our car was a stock model. Not only that, but the car was stationary, so the car itself didn't get to defend.

Well yea, in that case you are going to feel it. Car loses 4 armor from APDS but characters still get to add the car armor to their own. I wouldn't double penalize them by also applying the APDS mod to their armor as well but other GMs might. The dodge roll is for the characters in the car. Though it isn't in the rules I don't allow stationary cars to dodge, similar to how an immobilized character couldn't dodge (common sense call there).
Your stock car would therefore be easy to punch through with an SMG. The shooters are still suffering a -6 to pool though so you are more likely to dodge the shots. Hope this helped.

If you were really hiding in the car, and had your head down so you couldn't see the baddies, you wouldn't get a dodge.

Glad it wasn't me. Swiss cheese is swiss cheese; even looking at the bad guys there would have been no dodge test. I would add the car body to the pc armor though.