If I plug in my phone most every time I'm driving and don't allow it to fully charge, am I hurting the life of the battery?
No, or not in any bad way. You get the longest overall battery life if the average charge is around 50%. A smaller charge "span", eg. 75% - 25%, can give four times the number of charge cycles than 100%-0%, for the same level of capacity loss. Holding a lithium battery at full charge for long periods ages it far faster than when it is part-charged. It's not so bad in phones as with laptops, as laptops tend to also run quite hot and that accelerates the aging effect.
1- the power it consumes is very very small no matter for a car battery 2- most phones cut off the power when they were charged 3- it is good for a battery to charge and discharge .... if no charge it will be damaged that why the batteries have a USABLE BEFORE date extra note the killer of a battery is, pulling too much current ... in winter a car parked outside all night and in the morning you ignite ... the starter pulls more than 200amperes .. and this is bad for the battery