Digital Volt Meter Wiring Question

I would like some help on wiring a switch to be able to
check my battery pack voltage and also be able to check the aux.
12v voltage with the same digital meter in my dash.
I just want to be sure on wiring it up before burning something up.
Already have the meter (12-99) volts digital, have not got a switch
yet , need to know what kind to get first . I am thinking just a 3 terminal
On-Off-On (SPDT) will work but need to be sure, I guess the different
voltages is what's got me confused on how to wire it up.

Thanks for any help in advance.

Randy