I too had looked at doing 5.1 in my truck but then opted out, in fact I still have the Alpine centre channel speaker sitting on the shelf. I guess the fact that I would seldom watch movies in my truck and 99% of all music is only two channel, it lost it's shine. I decided to build an SQ install instead.
If you have considered all of this and still want to do it (maybe just because you can so why not?) I think it would be much easier to do with the sound card settings. It makes no sense to physically switch speakers on and off without changing the signal coming from the source so it will be mixed appropriately for the speakers that are turned on.
Also if you want an accurate sound stage the tweeters should be within a few inches of the main front drivers, not 2 feet away up in the dash.