One way to identify what is where without the hassle of drilling test holes is to just examine existing content.
Most all manufacturers include a drain at the bottom of the HVAC (Heating, Ventilation, Air Conditioning) system. And that drain may be easy to identify on both sides of the steel dash panel.
In this photo, taken from beneath the engine side of the dash panel, the drain is the little stubby molded plastic tube. If your manufacturer offered Air Conditioning as an option, I guarantee, the drain will be there somewhere. In high heat and high humidity, that drain can easily produce four or five cups of condensate (water) per hour (if the operator doesn't have the car in recirculation mode).
On the inside of the car, the drain will be at the bottom of the air handling unit, right beneath the A/C evaporator core.
You can use that location and measure up/down/left/right to determine exactly where to place your pass thru electric cables.
Note, you may well find extra holes/slots in the dash panel in unexpected locations generally sealed with a plastic / rubber plug. Those holes are used to accurately locate the sheet metal in the manufacturing process.