banging my head, 1 wire, 2 functions

I don't know how involved you are willing to get with this, but I have another idea that involves using an op amp as a two voltage followers.  The outputs of the op amps can then drive the transistors above.

Have you tried the relay diagram with the diodes?  If that works that would be quite a bit simpler.

Multiplexing gets a bit more involved and can be done in several ways.  For a voltage multiplex you can use a microcontroller with analog to digital converter or you can use op amps set up as voltage comparators.  I've personally done it both ways and it worked both ways.  When I went the op amp route I used to op amps for each multiplex input and created a voltage "window".

Since you are only using battery voltage and ground I think it can be done with one op amp chip (with multiple op amps on it).  I'll try to draw something up later today.

I would give this a shot.  The op amp (available at Radioshack stores) should provide isolation between the transistor bases.  The op amp will be set up as a "voltage follower" on both channels and will output whatever  you put in to it.  So, Pin 1 and Pin 7 will mimic your wire coming from the steering wheel.  The seperate, isolated channels should stop any feedback.