The problem is that most of the car electric is wired the way that connection is done switching ground (chassis) to the consumer.
I would not recommend high side switching using transistors because of resistive losses. (P-channel MosFets generally have a higher RDS(ON) than N-channel types)
When switching low side you must remove all chassis connections and move them to the drain of an N-channel MosFet.
My suggestion: Stick to available good quality relays.
Boncuk