2000 Freightliner FLD120 few small issues

Ck ground on rear end and the connection where the wire Hooks to the sensor on the back of the rear end

 

If it's a late 80s early 90s Pete on the fire wall there's a valve that shuts water off and on the is controlled by a cable follow the heater hoses u will find it mite not b opening all the way or ur heater core is stopped up
Good luck

 

There was a tread on here the other day I was reading about the same heating problems u got u mite ck the other post and see if u can find it
All I know much about is 359ex and 379ex Peterbilt's
Good luck

 

The rear end temp problem is most likely just from snow ice or water grounding the wire out.

 

On the heater issue. What is his coolant temperature. If it is low the air coming out the vent will be cooler. Is the engine fan on all the time? If he is running in cold temperatures he may need a winter front to prevent his radiator from getting to cold. But to much coverage can cause it to run hot.
On the rear end issue you need to you a temperature gun and see if the rears are really that hot. He can feel the front and compare it to the rear and see. I have seen people put mismatched tires on a truck (one tandem higher than the other) and it will cause one rear to run hotter.

 

Is the vent open to allow outside air into cabin. No flow when stopped. Flowing when moving?