Heat/ Air Fan Comes On Automatically

[mbh]

My 1996 ES 300 heat / air dash console is all electronic. My heating and air fan comes on randomly while driving. It comes on full fan (scary at times). The heat and air both work fine. When it comes on , and I attempt to turn in off, and it will come on automatically (I can't turn off unless I manually hold my finger on the OFF button). I wedge a credit card on OFF Button crack to keep off. If I restart car, it may not come on, but sure enough on a longer drive (say 30-40 min), it is likley to come on at some point.

It does not always do this. Only sometimes, and I can't seem to troublehoot when, or underwhat conditions this occurs (ie: certain speed or maneuver, outside temperature, etc).

A friend of mine said maybe check the ESU or ECU? Help.