1991 toyota camry

I wouldn't worry about it... better they run than not... If you figure out what the problem is and the solution is too expensive, consider wiring the fan directly to 12volts and installing an on/off switch inside. It'll cost you a whooping 5$ for the switch. I did that in a previous car. It worked well but I let it run pretty much all the time.

switch bad...

That should be a sign of a defective temperature sending unit. My guess is that they're designed to fail in the "ON" position for safety. I believe on mine, the sending unit is in the lower radiator tank with wires running up to the fans.

They also run if the AC is on, is theAC switch pushed in.