The problem is that when using a grid system it is harder to justify them not having a bonus when they can literally say "I literally have a car between myself and the shooter", not to mention it punishes complex actions even more then they already are. You already cannot do anything else, even move, and now you can't even take cover? On another note, if you were to use this houserule, do you feel these numbers are right?
So many literallies...haha.
Keep in mind that if the car is sufficiently large enough to completely obscure LoS, not only is the attacker suffering -6 to their DP (blind fire), the defender still gets to roll 4 dice to defend against the attack (unaware behind full cover), and the attack needs to penetrate the car and hit the target. Even if the attack hits, most weapons are not going to do more than tickle an equipped runner. Now heavy armor piercing weapons are another story but for good reason. As others have pointed out you can still move, no reason to repeat that part.

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As for size modifiers, I'm surprised nobody has pointed out that
SR4 Arsenal had rules for just that.
Micro Target: Micro-sized targets such as insects and micro-drones are incredibly difficult to hit, especially at range. Apply a –6 dice pool modifier. At certain ranges, these targets may not even be visible without magnifi cation (meaning the
Target Hidden Blind Fire modifier
p. 141, SR4 p. 178 SR5, would apply as well).
Mini Target: Mini targets include minidrones, very large insects, small rodents, and similar small animals. Apply a –4 dice pool modifier.
Small Target: Small targets include small drones, cats, small dogs, babies and toddlers, and similar small creatures. Apply a –2 dice pool modifier.
Large Targets: Large targets such as car-sized and larger vehicles, some orks, most trolls, big trees, horses, and similar bulky creatures are easier to hit. As a general rule, anything with a Body of 8–14 counts as large. Apply a +1 dice pool modifier to the attack.
Massive Targets: Really, really big targets are rare, but hitting them can be as easy as shooting the side of a barn. This includes things like buildings, trucks, hovercraft , aircraft , and large creatures like dragons. As a general rule, anything with a Body of 15+ counts as [massive]. Apply a dice pool modifier of +2 or more, as appropriate.
On that note, I'd like to point out babies and toddlers as targets... What a morbid wet-work assignment that would be! (0_0)