Is parked an adjective?
Yes it can be an adjective:
We walked past the parked cars.
Adjectives describe people, places or things. In the given sentence only two words describe people or places or things. The phrase "behind the garage" indicates where the truck was parked. That makes it a adverbial. The verbial "was parked" is a past perfect form of a verb. The word truck is the subject ( a simple noun) of the sentence. That leaves the words the red. Both the and red describe the subject truck. (Which…
As long as the parked vehicle is parked properly and not illegally parked in any manner, then the vehicle that rear-ended the parked car is at fault. Now if the parked car is sitting illegally (such as double parked or parked in a no parking zone, etc.) then the parked car is at fault or even both the parked car AND the car that hits it are BOTH at fault.