It does need a thermostat if this is the typical reading after 20 min of running in non-freezing weather. I assume you have checked the power at the aux. pump plug and found none after it warmed up. The system does have a shut off relay that won't allow the heating system to work until the engine is 125 degrees. I am not sure where it is located on the newer cars but it might be under the dash on the passenger side. That was where I found it in a 1977 280se. I would replace the thermostat, flush the radiator and check for power at the aux pump.

__________________
1977 300D Lost coolant while someone else was driving
1983 300D Can't run without oil
1985 300SD (gone but not forgotten)
1990 300TE 4matic Sold
1991 Yamaha Venture
1975 Kawsaki 250 triple
1974 Honda 200CL
1951 8N Ford
2008 Wildfire 650C