Whoops.... No SPEEDO and my bus goes faster

[Oonrahnjay]

     As an aside, if you only want a small 12V feed for limited periods of time (esp. while the engine is running), you can run a wire from the terminal that's on the "alternator hot" side of one battery and another wire (ground) to the negative post of that same battery (i.e. treat it like it is a free-standing 12V battery.)  Unless you have an equalizer, you shouldn't run it long or pull much power out of it in this way to avoid the problem on battery charge imbalance.  But you can use it for limited amperages while the engine is running OK.  Put a fuse that's rated about 30-40% higher than your rated load (i.e. if you want 3Amp for a GPS, a 5Amp fuse would be good) in line with the power wire.
     As I said, limit this.  If you need a temp 12V source, that will do it.  But keep it small loads for a short period of time.  And always disconnect it before you shut off the engine (in fact, I'd even disconnect it at the battery source when the engine isn't running even if anything attached to it is switched off but that's probably overkill).  It will work for you, though, for testing with a 12V GPS, or running a 12V trouble light (if that's all you have) or other similar things.

[Zephod]

I notice the strobe switch is available. That looks promising for a hook up. I  could attach the USB charging socket I just got at auto zone.