I dont think you will need logic for this but just an extra switch for the "prevent the spots being on at the same time as fogs " adding transistor switches will require just as much rewiring, plus then a CONTROLer device
A NPN transistor works so the "light you are switching ground" is connected to the collector, the emitter goes to "circuit ground" and the base commands the switch(usually with a lower voltage)
Transistors are a bit complex, it s better to learn the basics first, ie. ohms law, curchovs law, and such, before trying to understand and calc. GAIN.... and TOLERANCE
here is a good starter tutorial:
http://www.kpsec.freeuk.com/trancirc.htm
for 300$ I got an first yr cd rom from a local college,
heres one for only 10$ :**broken link removed**
(i dont know if this is actually what you want)
I found it hard to understand your description of what you actually want(in the end)(sry bad eng, if you could simplify?) If schematic 1 is wired ideal if thats how you want it to work, but schematic 2 gives me spins!! pls re-specify spot lights role!
turning on YOUR transistor would cause FOG and BRIGHT to power similar, ignoring the biasing faults, and the bad wires at the base,,
MAY I suggest using a RELAY in diagram, , way simpler for you to explain your methodology!