richaham

when its real cold the interior of my truck does not get hot enough especially the floor area and he rear.

ML 500 mz

Are you getting any heat? If so, is it only coming from center vents? If that's the case, colud be vent actuator needs to be reset. To do this you will need to *start the vehicle * press and hold defrost and air recirulation buttons at the same time for about 5 seconds then release. If things are in working order, linkage not broken, actuator should reset.
Hope this helps.