1998 freightliner det60 radaitor fan stays on full time. Where to start?

Is the radiator fan switch turned on? That's where I would start. I would also think it could be the thermostat.

As the Rev said, make sure your fan over ride switch is not turned on.

Some other causes of the fan being "on" constantly:
Fan temp switch in water manifold defective (stuck on).
Fuse or circuit breaker in the fan control circuit is blown, tripped, defective or missing.
Fan over ride switch defective.
Pressure switch in air conditioning receiver dryer defective.
A broken wire anywhere in the fan control circuit (the system reverts to a default mode of constant on if there is no voltage present at the solenoid operated air valve which controls the fan).
A defective solenoid controlled air valve.
No air to the fan control system ( the system also reverts to a default mode of constant on if there is no air pressure getting to the system).

Defective fan clutch.

How is the engine water temp? If the engine is constantly overheating for some reason then the fan temp sensor will tell the fan to run continuously.

If the fan is on even when the engine is cold, then it is probably one of the causes listed above

check fuse for fan. will be in with all other fuses.if blown it will cause fan to run all the time. 2nd if the solenoid fan switch goes out, the fan will run constantly. the solenoid will have an air lin running to the front top of fan area.