Being it is only when it is pluged in, my guess would be cold water leak & it may be at the heater hose conn left side of eng.
the good news easy to fine out. Look where the rubber meets the metal tube's not just for a wet spot of anti freeze But white "tracks" . can be checked warm or cold. Get a small mirror and lok on the under side of the connection-will be a thin white streak or spot where it drops from.
Next place is look at the end of the drain tube from the heater drip pan,but if it were seeping there you should get a sweet smell in the cab.
Not so nice a place $$ check the deck (under the head) gasket area, may seep out the gasket most of the time will run along the deck & gskt and down the back left (pass side) of the eng. have seen a few with this. and look for anti freeze or white track build up rear l. side of the block just above the pan--as i remember there is a boss for a transmission bolt it likes to collect on.
You may find the white stuff built up around a head bolt also. Seen that also,but if some one is like my b/inlaw-seeping valve cover gskts and oily--- hard to find tracks.
Being it is about the transmission & rear of eng has to be one of the three sources.
The tell tale tracks are a little off white and some times just a fine line,& have seen them large like what you might find on a batt term. conn.
As far as only happens when it is pluged in,you can bet it is seeping between cold and hot,just dry's aor blowen away running down the road.
another trick is tonight wrap alum foil ar plastic wrap around the easy to get to heater hoses to night-plug in and tomorrow check for the green stuf in the wrap instead of on the ground. .
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