Well, a pre-amp in a home stereo does more. It brings all inputs to the same level (and does riaa correction on the phono-in) and then also controls volume and basic equalizer functions.
For what you want to do, if the level of the 2 sources is controllable, just a line-level switch will do. You can find them in most consumer-audio stores, for people that want to add one or more inputs to their home stereo pre-amp.
As these are passive elements, you won't have troubles in the car.