Yes, Isolate your frog!
The crossing should have 4 isolating fishplates - on both blades, and on the two rails leading out from the formation. You'll need a dropper from the crossing to the common contact of your polarity switch.
The stock rails are fed as normal, and beyond the formation, the other two rails should also be fed from the same supplies as the stock rails.
Wires from your stock rail feeds should go to the other two contacts of your polarity switch.
The blades should be wired to the adjacent stock rail.
This would be easier if I were on a computer rather than a pad - I could attach a diagram.
If your loco stops with a short on the crossing when you try it out, simply swap the feeds to the switch.
This is also perfectly good wiring for analog/non-DCC
Good luck
Simon