TB, just cause a stock car says 6 speaker system, doesn't mean the HU has 6 channels. All it means, it has 2 front, 2 tweeters, 2 rears for a total of 6. That is how most car manufacturers describe 6 speaker setup. Some offer slightly different configuration, but they're still 4 channel HU. If it's not, they will clearly state it (surround sound setup).
OK, maybe this will help. My system has 3-way 6x9's in the rear, 2-way 5" rds in the front, and 2 subs in the trunk. Literally, this is a 12 speaker system!! Obviously there aren't 12 channels, but the way this is done is one signal is passed through a crossover that splits up the signal bassed on a certain frequency range. These multiple signals are directed to the appropriate speakers that can handle those frequencies. Capice?
TB, 3.5" is a LOW wattage speakers. They probably used 2 8ohm pairs in parallel for 4ohm load for the front. I bet if you put your head close to the 3.5 and 5 1/4" speakers, they'd produce same sound/music. You can practically treat them the same cause the 2 front channels are supplying all the power.