Make a paste of baking soda and rubbing alcohol. Make sure the unit is running and then smear that paste all over the location you want to drill. INSIDE and OUT!
The heat difference will cause the alcohol to dry quicker or slower where the lines run under the skin of the outside and inside.
Or, if you have any firefighter friends, take it to the fire house and plug it in and let them use their nifty infared heat cameras to exactly pinpoint the location of each line!