Ever hear the phrase potential difference? The problem you have is the voltage is not the absolute level of the point, but the difference in electrical potential between that and something else, usually ground.
Think of 12 volt batteries. If you wire them in series each battery still has 12V across it, but together they have 24V. But if you have 2 batteries completely unconnected from each other and you try and measure the difference between the positive of one and the negative of the other you'll read 0, not because there is no difference but becuase they're un-relatable.
The Idea of ground in a circuit is simply the place we expect all the electrons to flow from/to; its name comes from the actual ground, which you can stick a big old peice of metal into to use. But it's not that it has zero potential, it's that we're using it as our reference point and for our circuit any greater voltage is positive and any lesser is negative. You could call the positiver terminal of your battery ground, and the negative terminal -12V if you liked; it matters not unless you go swapping the cables round!
The problem here is that two circuit's ground (or reference voltage) can be quite different. One's can be at a singificantly higher potential than the other, often DC supplies are isloated from their AC sources; so two supplies might both run from the same mains (or even the same socket) but there's actually no direct connection between the reference voltages. And because they're generated from 110/220V there's no reason why you're 5V supply couldn't be the 5 volts between 100V and 105V, and the other being -20 and -15V. Hence you can get quite the tingle if you personally make the connection, and you can quite badly damage electronics.
The soloution is just to tie all your grounds together, preferably before you turn it all on (otherwise it can spark a bit). The direct connection between the ground will force the voltages to be relative to the same ground and the problem goes away. For extra caution you can do this and measure all the voltages before plugging in the teensy, and it's a good idea not to run the connections through the teensy - dont use the teensys ground pin to tie all the supplies together, use the breadboard.
From then on you can use the teensy's usb to communicate with your pc to your hearts content, subject to the maximum baud rate.
When interfacing the teensy to other devices, be sure to check that the device wont send to great a voltage to the teensy's pins. You'll need to check the datasheet of the dashboard to see what voltage level is used for its serial. If it's greater than 5, you will have to "level shift" between the teensy and the dash.