What do you mean you mean your wiring is 12/2? WHAT wiring?
Is there a circuit existing? Are you running new?

Your incoming 12/2 has both black and white wires hot. The thermostat may have labeling on the back showing Line( or L1 and L2) and Load. Line or L1 and L2 is where the 12/2 connects to the thermostat, Load is where the thermostat connects to the heater. Connect the black or white incoming to the red or black that says Line. It doesn't matter if you mix up the black/white incoming with the two incoming wires of the thermostat (usually labelled Line or L1 and L2). of the thermostat. Then the two outgoing wires from the thermostat connect to the 2 wires of the heater. Again, mixing colors doesn't matter. Ground wires are all connected together and to the thermostat's box.

i have it connected, and it is working. thanks for suggestions.